My mechanic told me that engines in Grand Dink 250, your People 250 and my 250 S are different.
Allegedly, S gives the highest output.
He also said what the differences are (bore, stroke, camshaft, valves etc.)
According to KYMCO bore and stroke are the same. Go here:
http://www.wantfacts.com/clients/kymco/index.php?ProdType=MOT&TrimId=26445 (http://www.wantfacts.com/clients/kymco/index.php?ProdType=MOT&TrimId=26445)
and use the comparison feature to prove it to yourself. (Yes, can compare Kymco to Kymco.)
Camshaft, valves, etc. may be different although I doubt it. Any changes would require that each engine type would have to be EPA certified separately. A costly process when you consider the number of markets the 250 is sold in. Final gearing is most certainly different since the GV has 12" wheels and the People 250's have 16's. It may not fit the People simply due to space considerations.
As for the additional 50cc's being a big improvement, it is possible. As an example, back in the '60's it was discovered that over boring the Corvair engine by .030" had almost no effect while .035" of over bore made a huge difference. The reason was that the smaller bore left the valves shrouded by the cylinder walls. The extra .005 improved valve flow and turbulence. Weather that's the case here, I wouldn't know.
Another thing to consider. Reading the ad, I notice that they recommend changing the cam ($90.00) and the exhaust ($430.00) on top of the $230.00 for the over bore kit and this gets kinda pricey.
My last comment is an old engine builders quote, "The faster they go, the faster they blow."
My vote? Unless your Grandvista is just a sometimes short haul toy:Not Recommended! Not Recommended! Not Recommended! Not Recommended! Not Recommended!