I gained about 2 mph going from 35 cc to 28 cc on a hybrid. They weren't even mountain bike tires - just the bigger hybrid kind - and 28 isn't even that small.

Tell PITA it's your butt, it's your bike, it's your choice, you love him dearly :-) (Or wahtever fits... it is yhour PITA too :-)) ... of course, by PITA you mean perplexing, intelligent, tender adult, right?