I love the idea of corruption gear effects replacing the dying part of surrender to madness, however I strongly oppose baking negative effects into the core spec.
The way risk/reward system works is through the individual’s choice to risk for the rewards. This is were the mental games and excitement lays. If you remove the individuals ability to choose whether to risk it for the reward then it is not a risk/reward system anymore it turns into beating the odds system.
you have a set destination that you need to reach and as you progress to it the odds get stacked against you. in this model it would be that you are generating insanity as a core part of your spec mechanic however by doing that you will get negative effects stacked against you.
this is why having negative debuffs should be locked behind a cd because the whole excitement behind it is whether you should take the choice to risk it or not.
my suggestion is something similar to what Yvaelle had proposed.
A heat/build up/tier type of mechanic. There will be a fixed drain rate that would increase slightly with each tier (say 3insanity drain per sec + 1insanity per tier). there will be 5 tiers in total that break down into:
tier 1: insanity between 1-20 (3insanity drain per sec)
tier 2: insanity between 20-40 (4insanity drain per sec)
tier 3: insanity between 40-60 (5insanity drain per sec)
tier 4: insanity between 60-80 (6insanity drain per sec)
tier 5: insanity between 80-100 (7insanity drain per sec)
each tier will give you %3 haste and damage buff to a total of %15 haste and damage buff.
think of it as you start in voidform rather then building up to enter voidform.
Voidform becomes a 1min cd for 15sec that will lock the drain in the tier you are in when you caste it but allows you to go into the next tier. so if you are in tier 3 and you cast voidform your insanity will not drain below 40 but you can generate more insanity to get into tier 4 and 5.
surrender to madness if chosen would modify Voidform( maybe add more haste or crit or another additional buff that has to match the risk factor) and take its place implementing your idea of risk/reward using the corruption gear negative effect.