There appears to be a diminishing returns formula here where your teams level, postive and negative traits, and gear will improve your chance of success less and less the more you get. It seems highly asymptotic in the high 80's/low 90's. This makes sense since this is the way bonuses are handled in game, especially MP. It's for this reason that you can get more than 100% reduction in cooldown for some abilities (Charge, in particular), yet not have a 0s cooldown.
I do not think your plan of buying +6 effectiveness gear will help you: 1) because you're going need more than 6 to get 6%, and 2) you can't use more than one piece of gear.
