I came across this thread rather by accident, but I thought I'd jump in and answer a few questions. 40hp? Almost. Haven't had a true 250 make 40 yet but I've had them in the 39-39.5 range. As for a stocker making 36hp, not on my dyno they don't. Pre-'06 make around 32hp on pump gas, that's with the proper jetting. (leaned out a bit) Add 1 hp for 2006 models from what I've seen, the head is slightly better. So as someone said, that's around +7hp over stock, which is the correct way of interpreting dyno charts. I don't get into "numbers wars" I prefer to look at the delta instead. Squeezing 7hp more out of a YZ250F isn't all that easy, it takes a certain combination of parts and a lot of tuning to make it all work as intended. There is a lot of hype out there, so you have to take everything with a grain of salt that you read.
Reliability: Pretty good. In expert hands they will run a full season, but it takes a mid-season cam chain swap, rings, and new valve springs. I have had a smidgen of trouble with camshaft journal wear on the exhaust side but I should have billet cams available sometime soon to rectify that. You have to watch the valves closely. (I use OEM) After 1 season it is time for a total rebuild from the crank up.