I have MUCH experience with early XL250s and exhausts. I raced enduros, flat and dirt tracks, MX, TT, and scrambles with XL250s, and rode one from Tennessee to Alaska and back. The first XL250 carbs were pre-EPA and were 2-3 mains richer than what is sold today, but any of those pipes would have required rejetting, maybe richening the midrange, but definately leaning the top end. Note that ALL the aftermarket pipes peaked and were losing horsepower at 7000rpm, and the stock pipe was still gaining at 7000 rpm. The red zone on these bikes started at 8000rpm. Extrapolate the horsepower curves and the clear horsepower winner is, drumroll please, the stock pipe. Make even moderate performance upgrades to the engine, and the difference would be even more apparent because all the aftermarket pipes in this test were already used up.
Anything close to street legal with a USDA spark arrestor took weight off but made little difference in performance, or were worse than stock performance-wise. The only two aftermarket pipes that could hang with the stocker on a modified engine were from Kerker and Rocky. Both were virtually identical to the stocker with their baffles installed, but made definite gains on top when the baffles were removed. Both also saved about 8 pounds of weight. Take an XL250 out to 340cc with a Powroll stroker and big bore kit, port and polish the head with oversize valves, good carb and cam, and the Rocky without baffles walked the dog. 4 good 'ol boys on 4 XL340s took top 4 places in 4 dirttrack races on 4 consecutive Fridays, 4 Enduros on 4 consecutive Saturdays, and 4 MX races on 4 consecutive Sundays, and the local sanctioning organization banned stroker cranks.