100% is 100% in this case. Basically whatever damage the ability does that applies the walking bomb is applied as a walking bomb dot on target. So a flame array firemine that does say 20K damage applies a 20K spirit dot +/- 10% which is just random variation in the attack script. A 40K fire mine applies a 40 K spirit dot lol if you can get that high. There is one actually two things that modify this in a beneficial way: first, your critical damage bonus applies to all hits and the explosion of walking bomb (the final dot tic) is a "hit". So if it crits and you have a say 150% crit damage bonus you can multiply that up, I did get a 45K spirit damage hit from a walking bomb that way once. Secondary on, necromancers, the masterwork DOES gain the virulent property of the actual spell, so it can spread like the upgraded necromancer ability (if a target dies having a say 20K spirit tic from the walking bomb masterwork, the 20K dot spreads to nearby targets). But it is still 100% damage, just like the masterwork says. Those damage "upgrades" like 500% or 900% from the actual ability have nothing to do with the masterwork. Oh, and lastly on necromancer you can manually detonate a walking bomb applied by the masterwork at any time and it does not cost mana or trigger a spell cooldown on it unless you actually cast the ability as normal.
Anyways I did extensive testing and soloed nm+trials and trespasser with such a necromancer and the damage in aoe is spectacular: frost array, fire array (applies walking bomb masterwork), everything unfreezes and dies. But my impression that those 500/900% bonuses from the ability toggles do not apply to the damage, only the virulent property which is still a BIG bonus against mob groups, but your critical damage bonus applies to any walking bomb explosion as long as it crits. Still a 20K flame array firemine is pretty good...
Anyways you be the judge wtf actually happens:
EDIT: Given the way the effect works it may be better to equip a SUPERB Critical damage ring and not enhanced walking bomb ring but ONLY for the masterwork effect but I have not tested if enhanced walking bomb ring applies to the masterworked walking bomb or not, I suspect it doe not as 100% seems 100% no more no less BUT I have not tested it.
On normaly cast walking bomb the enhanced ring is great because it actually increases the damage by 40% or so of the total so 500% becomes 666% on one upgrade and 900% becomes 1300% or so on the other.
The biggest advantage of a necromancer when it comes to damage over the other two mage specs, which only becomes apparent if you play around with the ability combinations enough is that a necromancer mage can be switched from an inferno pact fire specialist to a lightning storm pact specialist on the fly without loosing any effectiveness or giving up points in the skill trees one way or the other. With the fire belt setup you can do the flame array spam as seen above to trigger your massive walking bombs. If you switch to storm pact in case you are in a fight with lots of rage demons etc or against fire resistant promoted enemies or fire dragons you can instead unleash lightning cage+ walking bomb + double energy barrage sequences (with electric staff) all day long. Some battles are still "easier" to do at some distance for more safety especially under nm+all trials mode solo, that is why being able to switch to a lightning distance mage specialist on the fly is very helpful, and although I did most of the game solo with the inferno heavy build, I switched to a lightning setup against the final boss for instance, mainly due to the hardest part of that battle being when the rage demon and the fire dot whisps show up.