I've upped the gearing to 15:50 for road use and gone back to the 32mm internal diameter muffler outlet. The bike tops out at around 140kph now by the speedo (felt faster) but I'll calibrate the speedo using GPS as standard soon.
I'm considering going to a 52 rear permanently and just changing between 14 and 15 for the gearbox sprocket with 14:52 for off road and 15:52 for the road.
A permanent 51 rear and changing between 14 and 15 could also be useful - I know it would be OK for the road but only a run in local off road conditions will tell me if 14:51 is low enough for deep dirt. I suspect 14:52 would be just about right for the torque this thing has running a set of knobbies.
I'm still blown away by how hard this little bike goes now from 50% to 100% throttle. A dyno run would be interesting .......