I recently installed the 300cc upgrade. But my scooter came to me broken, frozen rings, scored cylinder, blown head gasket, etc. I figured it would be about the same price to upgrade, rather than boring and hunting for the right piston, etc. So I don't know if it improved performance, as I didn't get to ride it before the upgrade. My searching showed no differences in the kymco 250 engines that I could find. Mine says "KHE7", and from what I could tell, parts are parts... I had bent valves too... just ordered parts for "kymco 250 khe7". Everything fit perfect. External stuff like exhaust pipe and air filters, etc. might be different due to body style alterations, but everything I looked at before doing this showed all the engines are the same internally.
The 300cc kit was a perfect swap out, nothing extra needing done. I can tell my carb needs some minor attention, but I fiddled with all the screws while it was apart, so that's expected. Don't know what to say beyond that. I haven't taken it past 45mph yet. I'm new to 2 wheels, and it's still a bit spooky when the wind gusts hit. lol. Wish I had another 250 to compare it with.