We used to cap each pool at 100 and now we don't anymore. Shifting members with 100+ up into the top pool resulted in above average SP levels in the next few pools that had been stuffed with members at 100. That still happens a little, but less of an issue now.
As each pool independently targets 2 SP per unit, having some of the pools above average does not actually take any value away from the pools that are not. We are a lot more mindful of keeping pools from being below average for more than a couple of days at a time.
It's all sort of academic, since the automated system is designed not to have these issues. There won't even by explicit pool assignments. Instead, all members will receive a specified daily rshares per unit of SBI and the best account to deliver each upvote will be identified dynamically when they post.