a few yrs back when i swapped over to an aftermarket straight through exhaust (with larger piping), from the ori straight flow one, low end did suck quite a fair bit, but after a coupla days (or weeks, cant remember now) it felt better throughout the powerband and showed gains over stock (negligible low end loss,measured using a gtech meter)...and it did sound a lot louder than when i got it
dunno why though, fibers in the exhaust slitely restricting the path? then burnt off? hehe could be a possibility....
or mebbe a simple retune of the carb (if ure using one) wud help