Yeah, 'sweet spot' sorta implies that there's such a thing as overkill. Can't never have too much ram. Never seen anyone selling a 7200 stock hd, but if you were really hell-bent on having one covered under warranty, I believe the techs at an apple store can install one without voiding the warranty.
On the other hand, check out the benchmarks for 5400 vs. 7200 rpm hds. You can't get a 7200 at 80 gig (if you find one, lemme know...) and the speed increase of the 7200 over the 5400 isn't that much.