First thing First..
5th gear is for Cruising it really does put a load of pressure on engine by which a redlining engien in 4th gears stalls back to 8k RPM (remember piaggio outputs its peak horse power at 8~8.5k RPM) so dont worry its making a complete power at 5th gear... but if it continues to go down more that means there is a sure power / compression loss...
now, how to make it go beyond 8+ RPM in 5th gear increase performance, like as I said better plug better furl, better ignition and the see it smilling at 11k on 5th gear given that you are not doubling or the road is not uphill e.t.c...