Too slow a twist can result in the heavier bullets not stabilizing properly and a tighter than required twistcan reduce the velocity potential slightly
The first part of this statement is absolutely concrete...it can't be argued.
The second part however is theoretically true only. I've never been able to measure it and I'm beginning to think the difference is less than the shot to shot variation of a given load in a given barrel and twist. In other words...it's marginal at best.