at that high of an RPM I wouldn't worry becasue as far as I know the engine has a soft limiter that starts making changes to reduce the engine's power as the hard limiter (where fuel is cut off) is approached. I suspect that what you are experiencing is this soft limiter.
the flow area of the 2, 1 7/8 pipes you have at your rear split provides over 2 times the flow area of the single pipe heading to the split.
