what problem?
there's no "problem" starting the yz250f hot or cold. i don't know where this "yama's 4 strokes are hard to start" myth got going but i've got an '01wr250f and my brother has the '01yz250f -- both start on the 1st or 2nd kick, hot or cold. in fact i rarely use the hot start button. a couple of weeks ago, there was a guy and his (140lb) 15 year old parked next to us at the track. the kid was already riding a yz250 but he and his dad were real interested in the yz250f. i gave the 15 year old thirty seconds of instruction and he started the 250f on the first kick, hot.
if you take 5 minutes out of your life to a) learn the "drill" and b) adjust the fuel screw correctly, these 250f's will light right up. many many wr's have needed fuel screw tweaks -- they are shipped from yama too rich. an easy test for this is whether the bike starts easier cold with the hot start button pulled out (the hot start button leans the mixture by introducing more air). if it does start easier, well, you need to adjust the fuel screw IN (=UP, since it's upside down on the bottom of the carb).
after my fuel screw adjustment, there have only been maybe two occasions which required more than 2-3 kicks to get it going. on one occasion i bit hard in some ugly sandy whoops, and the bike was on it's side for more than a minute while i shook the stars out of my head. in these cases, if it doesn't start on the 2nd or 3rd kick, i go into "clear it out mode", pull the decompression lever, and kick it through about 5-6 times. then restart "the drill" and it's alive.
the stock pipe is good. not great, but good. all these shootouts cater to the advertisers-- i don't think that there is more than a 1-3% difference in performance with any of these pipes compared to stock. of course, on a two stroke, the geometry of the pipe is everything, since the engine relies on the pipe for proper combustion chamber scavenging. but on a four stroke there is much much less dependency on pipe geometry. there's no "quick" 10% gain and you certainly can't "move" the power around much (e.g. more low or top end) like you can with a two stroke torque pipe/rev pipe. wanna know what the highest performance four stroke exhaust configuration is? go to the drag strip and look at what those guys use on 2500hp big block blown motors. nothing! 4" diameter open headers, about 10 inches long. but they'll make your ears ring at 100 yards.
the wrooster