I've noticed a small difference on rehydration. Then again, since I am typically doing fairly small batches in the 2.5-3.5G size until fairly recently (when I've been doing more 4-5G batches), the fact that I've been direct pitching an entire packet of dry yeast, versus rehydrating probably didn't make much difference.
At 2.5G of RIS, I would highly recommend rehydrating. Supposedly direct pitching leads to 50% cell death initially. I'd say that number is BS, in that since the death is caused by shock from not controlling what is taken across the cell membrane at first, then the gravity of the worth has GOT to influence the cell death %'s, which means higher gravity is likely to lead to more initial cell death if you don't rehydrate.
Frankly rehydrating is minimal extra effort, so I have been the last few months.