if cut slightly, will increase max rpm possibly achieved, and will shift the powerband slightly to higher rpm range ex.: you have an OS TRPT that gain 1.5-ish HP @ 32.500 rpm & 42000 rpm (max) with 1.5 cm stinger length,... so by cutting it shorter to, say, 1 cm, you get that same 1.5 HP but at 38,000 rpm & 43,000 rpm (max) (these figures are not accurate, use as illustrations only).
so, shortening the stinger will (theoretically) skew the powerband in-favor of higher engine speed (rpm). you'd have flat powerband when at lower rpm, steep spike on the "on" spot. good for people that tend to squish the throttle more (hardly let go and brake)
longer stinger might fit the "butter finger" better, since it has peak power in lower (than normal) rpm. average driver will feel this kind of pipe as "wild on turns, and less speed on straights). I prefer this setup myself, since it's a lot easier to pass a guy in the turns rather than the straights (the one w/ more control and "feel" will win), on the straights, it's just the matter of how high you can go (speed) and how long is the straight (relates closely as well to how quick you achieve top speed).
oh, one other thing, cutting your stinger shorter than 1,5 cm will be deemed illegal on most big/organised/international races, but if your track allows anything I'd say, go with the biggest stinger diameter you can find, and play with the length (STS sells a pipe w/ replaceable stinger to suit your preference in its length), or make your own pipe, so you can ultimately decide what, where, and how (just like me