This discussion has brought up a lot of great points, but I'd like to suggest a simple framework for handling problems in discussions in future steps for CAP6.
1) Try to make CAP6 as good and powerful as possible, within reasonable bounds of the OU tier. Go nuts with offense, defense, speed, priority, etc -- make this as amazing as we can (within OU norms, of course).
BUT ALSO...
2) Try to make Belly Drum its *best* set.
My bet is that it is almost impossible to build an OVERPOWERED OU pokemon where Belly Drum is its best set. If the pokemon is *too good* by having too much attack or bulk or coverage or speed or whatever -- then it won't need to use Belly Drum -- it would be better to use it as a general wall, a coverage hitter, a revenge killer, etc. However we accomplish #2, if we can ensure that Belly Drum is its best set, I think we pretty much force the pokemon to be balanced.
Some people seem to think that we could build a pokemon that will be a Belly Drumming monstrosity ripping all of OU to shreds. My guess is if we do build such a pokemon, we will discover it is simply a monstrosity -- it will be much more reliable and versatile without hassling with all the risk/reward bullshit associated with Belly Drum. I don't think we should be nerfing aspects of this pokemon when we think it will be too good with Belly Drum, we should nerf aspects of this pokemon when we think it will cause the pokemon to NOT USE BELLY DRUM AT ALL.
This is a big distinction, and I think it will help regulate discussion on the project going forward.
If we get caught up in debating every claim of "It's broken!", we'll get nowhere on this project. Sure, we have to deal with accusations of brokenness on every CAP, but I think it will be even more pronounced on this CAP. We're going to be slinging around battle strategies involving a pokemon with a +6 Attack boost, which probably means it will have an effective attack stat of 1000 or more. Big numbers like that freak people out, and I'm sure the boo-birds will be out for blood. And I think it will be incredibly hard to discuss which claims are valid or not, because we have very little game experience with attacking power of this magnitude, coupled with bulk, speed, priority and all the rest.
I think we need a better benchmark for dealing with concerns that CAP6 is "too good" -- and I think that benchmark should be the following:
CAP6 is "too good" if Belly Drum is not its best set.
That is much easier to debate, IMO, than general claims of brokenness. Rather than having people point to all the pokemon KO'ed by CAP6 at +6, or pointing to how many hits it can take at 50%, and screaming "See, it's broken!" -- I think it is better to focus overpoweredness claims at CAP6 itself. Simply look at what the pokemon will be able to do WITHOUT Belly Drumming, and if it can do other things better, then we probably made the pokemon too good overall.
That is NOT to say Belly Drum should be its ONLY set. As Yilx explained very well, we probably want to give this some other sets to keep the opponents guessing, or for specialized use on certain teams. If we do it correctly, by giving this other sets we can actually make the Belly Drum set
better. But most OU pokemon have a set that stands out above the others, and for CAP6 the Belly Drum set is the #1 goal. If we do that, I think we'll avoid making an overpowered pokemon AND we'll achieve overall project success.