35 to 28... now *that* makes a huge difference.

25 to 23 - at that point, the other differences in the tread might make more difference. First off, some tires with the same size number just aren't the same size, I don't care what they say (like dress sizes). Other tread differences and how much pressure is in 'em will also make a difference. Chain "stretch" will make a difference - how many miles on the chain? Seat adjustment? Foot position? Stroke technique ("round and round, not up and down)?
Lots of things to think about for getting that edge... more chocolate or other caffeine source (or slip him decaf) would prob'ly do it :-)