Is my Piaggio NRG running as expected?

Having serviced/rebuilt my NRG I'm trying to figure out if the performance is what should be expected.. I have no other bike to compare it to, so need your opinions.

Spec:-

50cc standard barrel
4.8G rollers
Endy derestricted exhaust
Standard air filter
.64 main jet
Variator washer removed

Currently revs at 8500rpm on the 4.8G rollers, accelerates ok up to 30 on the flat.. after that it will continue to get to 37/40mph on the flat... downhill I've had it to 47mph and up a steep incline it will do 26-30mph...

Fuel economy seems to be about 40-50 miles from a tank (£5.80) .... which I work out about ~50mpg which I think seems a bit low?

Spark plug is a nice light brown colour which suggests the mixture is ok.. certainly seems to run ok, no flat spots or hesitation and keeps a constant 8.5k revs.

The exhaust sounds very noisy, which I'm not too happy about.

Does this sound about right for my setup?.. I'm thinking to change the exhaust as I've been reading that these Endy power pipes aren't very good..

 

I had and endy on my jog rr... really bad top sped and acceleration with same bad fuel economy. Went for another exhaust a polini and upjetted to 74. Scooter runned a lot better. There just no comparison between the two. Also my endy pipe had a smaller 1mm tube inside the main pipe where it connected to the engine. I guess thats a bad thing for it. If you can cut that part out you would see a noticeably improvement in overall performance.

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