I think assassin is better, especially when you get 9 seconds of nothing but pure critical hits when in stealth, might be better than tempest. You also get knockout bomb which seems extremely important in nightmare.
You also get hidden blades which you can use with cloak of shadows / so thats 6 hits at 300% extra damage which are also critical hits.
Tempest only has 2 things that seem good, slow time and thousand cuts. Nothing that helps your party in terms of survival though. Flask of frost seems like a death sentence for your rogue.
You know you can take elixir of frost without the upgrade that taunts everything right? so you take 85% less damage for 8 seconds and freeze anything close to you aswell, i literally have no idea how that could be a death sentence at all.
Also Hidden blades looks amazingly good when you look at it i agree and my initial plan was to go assassin however at best Hidden blades in my current gear (my gear isn't amazing) does about 900 damage per crit so over 6 hits thats around 5.4k damage, deathblow however at or below 50% of an enemies health hits twice and currently crits for around 3-3.5k per hit and going upto 4.5k+ at lower health, also with Elixir of fire you can get off 4-5 deathblows in 8 seconds.which at minimum is around 28k damage in 8 seconds.
Playing around with my companions and testing out the damage they can do (pretty sure the inquisitor would do more just based on stats) the only class that comes close and possibly beats Tempest rogue on pure burst damage is Reaver and that requires you to spam Dragon-Rage which is going to end up with you dying 
And that is purely talking about damage dealing, with elixir of frost, elixir of lightning and stealth a Tempest rogue can pick off hard to kill targets before they even act and control large battles very easily.
For example 2hand warrior type enemies have been mentioned alot in this thread and i can tell you i have not had a single problem with them since i unlocked Tempest, just open with elixir of lightning and burn them down before they can even react, you can quite literally run into a group of enemies are Tempest pop elixir of lightning pick off a few enemies pop stealth into elixir of fire and a group of 5-6 level 20 enemies will be dead by the time the elixir runs out.
Again this is mainly because of Deathblow being so strong for example a normal melee crit for me on a sundered enemy is around 300 damage, for some reason even though the base damage with the upgrade states it does 300% + 3% for every 1% health missing, instead of lets say a 1% health enemy taking a Deathblow doing 597% weapon damage which in my current gear would do about 1.8k the 3% multiplies the 300% somehow to the point where at 50% health my Deathblow is doing around 1000% weapon damage (3k crits vs 300 crits for normal melee) and because when an enemy is below 50% health Deathblow hits twice that actually puts Deathblows actual weapon damage at 2000%+ maybe even hitting 3000% when the enemy is low enough health.
I am currently trying to get a decent crit build that also has a good amount of armour pen for the dragons because if i can negate the armour they have I'm pretty sure i can kill a 200k health nightmare difficulty dragon in under a minute with a double tempest group.