I thought I read something that the issue with those CRF's was the head material being too soft, so the valves were 'seating' to the point of leakage.??? I dunno. It's late.
There were LOTS of complaints about valves on the intial batch of CRF's (03 450s and especially 04 250R/X's) alot of this had to do with the riders. This was a transition period, everyone riding track prior to 03 was on a 2 stroke, they revved the hell out of them... the transition to 4stroke high revving bikes was a big improvement in performance numbers... but what the riders didnt realize was the different riding style was needed as you DIDNT need to rev the piss out of the 4strokes to get power... as they had power throughout the rev range. This led to many of the early race 4 strokes being over revved and banged on the revlimiter MUCH more than they should have.
Also, Oil changes.
Again this was a transition period... the 2smoke riders had never heard of oil changes, at most maybe transmission oil every season... but god for bid changing the oil in a motorcycle every track day thats ludicrous... unfortunately not. The honda CRF450/250's have a quart or LESS in the engine side of the case (7/10ths of a quart for the 250x) that VERY SMALL amount of oil gets chewed up real quick at high revs and due to the small skirts there is some oil consumption. This led to bikes running bad oil as well as many oil starved bikes which leads to failures of ALL kinds, especially in the head (cam shaft n rocker wear).
Pair ALL of this together and you get excessive wear on the components within the head, and excessive valve seat wear along with valve cupping.
This is why, and im sticking to it, i do AT MOST 50 miles of track riding, and 150ish miles of trail riding before changing oil in the engine side. And i keep a constant eye on oil level (ill go through half a dipstick in a 40 mile track day, and its been like that since day one). Im not saying that i dont bang on the rev limiter from time to time... just watch any of my dirt videos and i hit it quite often... but not like in the beginning of the 2 smoke guys where it was CONSTANT.
And yes they did slightly change valve seat materials i want to say around 06.
Typically the valves will wear at the seats and the valve will travel further into the head thus getting tighter on the clearances to the point of zeroing out which will leave the valve open causing leakage and often a burnt valve.
BACK ON TOPIC, i do not have a tightening valve problem... which is the magical part, and why im not really worried about the exhausts being loose, Ill probably end up re-shimming down to .30 (so .02 on the loose side) to get things back within spec... but more input welcome.