Your math is a bit misleading there. For that 8 seconds (or less) Your only using one ability in repeated fashion. While Rift Mages are open to using other things in those same 8 seconds.
Those "other" things are not exclusive to the Rift Mage. The only non-focus damaging spell Rift Mage has is Stonefist. So during those 8 seconds, a Rift Mage can cast other spells, but a KE can cast those exact same spells.
You cast a spirit blade strike. You do 300% damage.
Where as I cast my stonefist which is going to do 500% damage plus add my Weaken affect. Which reduces incoming damage on my tank and increases my mana regeneration by a fair amount. Weaken has an issue right now that it does not apply to the hit that causes it. I'm ok with this.
Weaken effect only adds 25% damage (or so the wiki tells me and i'm out of game atm). The reduction in damage on the tank is insignificant. Tanks properly spec'd can take as little as 1 damage from the front. And this was before they Patch 5 which fixed most of the tanking passives. Besides, the only enemies that you really should worry about are stealthed mobs that you don't see coming (and hence you don't weaken) or boss targets. Most boss targets will put up guard and you can't cast weaken. And should anyone put "shocked' on a boss target, you won't get weaken. So no, this absolutely does not help the Rift Mage. At best I'd argue half the damage due to weaken being on the target half the time. So 500% * 1.125 every 8 seconds.
You say the Rift Mage is doing other things. So is the KE who could cast spirit blade 8 times in those 8 seconds. My whole comparison was with a lazy KE who only did it 4 times. Firemine...both could cast it before, or during. Same with immolate. All the damage compares, especially with the craptastic weakness debuff not working in tandem with the broken stonefist detonator.
Second second in time. You cast spirit blade. You do 300% damage. (it has now taken you two strikes to match my own basically)
I on the other hand am feeling a bit aggressive. Want to get some more affects up and my cursor is already in place. Having some fire I'm gonna do immolate with my non-optimal damage build but I have it upgraded. I now do 400-500% damage (forget which right now) But I also do a good 150% or so damage on future seconds for 8 seconds.
Note your till behind here. You did 300% to my 400-500% before the weakened damage is factored in.
3rd time around. You swing that sword again. that's your 300% damage. I on the other hand release Energy barrage.(again. I'm not optimized) 66% weapon damage over 12 hits which usually all hit. and upgraded I lower their resistance to an element for 24%. (but I get the feeling that some bosses may be immune to the resistance lowering. I haven't been paying attention to it quite honestly.) This puts your 300% against I believe it's like 760% damage if all my hits connect. and then on top of that I get my 150% burning damage to add to this second.
Against non-boss targets, anything is likely dead after 2 swings so the point is moot. Against boss targets they often have guard up. Spirit blade trumps guard by far. KE can cast spirit blade every second for all 8 seconds. Rift Mage is not mana-limited, but cooldown limited. So they use their Stonefist, and then their immolate, and maybe an energy barrage...oh crap, they are running out of spells.
Mean while the KE can do 4 swings of their spirit blade, toss out a immolate, toss out a firemine. Given chaotic focus and the fact that by doing damage, it fills their barrier back up, they are getting +50% damage on both immolate and firemine.
Oh, and if the boss target has guard up, then weakness can't be applied. if someone in your party puts "shocked' on a boss target (i.e. sleep immune) you can't apply weakness either.
Most Rift Mages will have PotA, veilstrike, Stonefist, focus, and 4 other abilities. Even if they completely ignored all support/utility spells (like barrier which gives the KE a +50% damage on firemine/immolate), they only have 5 damaging abilities to cast in those 8 seconds. The KE will have 8 seconds in which they can use spirit blade, which will match or surpass stonefist half the time on any target that matters. And when they aren't using spirit blade, they will toss out the exact same abilities the Rift Mage has. And if the Rift Mage has chaotic focus, just like the KE, he has to stop and re-cast barrier to take advantage of it. Furthermore if the Rift Mage gets aggro, he has to move, take a potion, or take steps to survive. The KE just casts spirit blade again...barrier full.
Now I've cast some big spells. I may need a second or two about here So I'm giving you a couple extra hits. So you'll get to pile in about 600% damage while I'm doing a piddly like 146% plus my 150% of burning is going to be about equal to your 300% for each second and I'm just autoattacking here. I think it is give or take without a calculator handy. It's just at this point here where you are catching up with me. But that's ok. This is what balance is about.
As I said the burning spells are not exclusive. The KE can match the RM fire spell per fire spell, but has the advantage with chaotic focus. If the RM is damn lucky he'll match the damage, and it all comes down to if he can apply, and maintain, the weakness debuff.
The more I think about it, the more it all comes down to that weakness debuff. Every other RM thread is complaining about that debuff. It's not dependable. It conflicts with shock. It doesn't work on the target if he has guard. So many problems. And the first time the RM spells don't work, he is either going to die or has to run away sucking up healing potions. Stonefist doesn't knock down bruisers, big demons, dragons, etc. KE just smiles and swings her blade.
But after this couple of seconds I can start releasing stuff on the enemy again. from my spell arsenal.
This is not even counting other passives. Just some basic up and up competition like your own math. Now you are going to regen mana faster than me. I'm also considering this against your average boss. Results can vary a bit against certain ones like fire resistant dragons. there you might start eeking out ahead. So I'm not nearly so gimped as a rift mage as you want everybody to believe. And I'm not as in your face and having to get certain attacks in to keep everything from killing me either so I have some warning if I have to vary and get off the boss to attack something else or drop an aoe on the tank which can switch up the math a bit too.
What other RM passive do you speak of? There is only 1 to boost damage. And it's not dependable. Every other one is not exclusive. And the biggest 2 I can think of (chaotic focus & dispel) favor the Knight Enchanter. Most Rift Mage builds don't delve too deep into spirit. And if they do, they don't have the damage spells they need to fill in those 8 seconds. Chaotic Focus is the trump card for the KE who fills her barrier after every attack.
I'm sorry but I have to disagree with you. I do admit the RM is fun. But on my RM I toned down the difficulty level to match the KE's potential.