I've got the stage 4 on my 400 and I love it. If i had to get another pipe i would definitely go with the stage 4 again. On mine it helped with power and torque pretty much all around. But if i had to pick where it helped the most, i would probably say low-mid. Mine doesnt seem to be as louder as other pipes though, its got a better tone i think. My brother has a big gun on his 400 and i really dont think it gave as much power as the stage 4. By far one of the best pipes around. Hope that helps.
"Go Big or Go Home"
'09 Ktm 450 sx
Old quad- 06 LTR 450
Older Quad-01 426ex
-98 Camaro v6, previously supercharged
-69 Camaro soon to be restored
-53 F-100 soon to be restored
FQX, Mx, FMX, aka all purpose track in the works, built by yours truly