Hi!
It might be that MVVS has altered the compression ratio on the latest 6.5-8.0cc MVVS engines.
This certainly applies to their Q-500 6.5cc engine which is made for 15% nitro (but MVVS has heads specially made for 0% nitro too).
As for the your expectaition regarding MVVS 8.0cc engine...I think it is by no means no more powerful than any other 8,0cc engine...! Simple as that.
As for the pipe outlet being to small...no... to the contrary, you can perhaps gain some rpm by decreasing it even more.
Did you have the 7mm carb? I would definitely try the 8mm carb as I have personal experiences from it on my 6.5-8.0cc MVVS engines.


Regards!
Jan K