Yes the 660/700 are night and day differance. They don't even compare. If it were me I would not mod the 350 and just save for a big bore quad. I use to have a warrior that had a intake, Cam, High Compression piston, Port and polish with oversize valves, and full exhaust and it was not even comparable to a 660 stock. Was alot faster then a normal 350 but nothing like the 660.I also noticed that the 350 aren't that popular. What gives? Makes me want to sell it now and save up for a 660 or 700. I mean are the 660 and 700 that much more powerful, and funnier to ride?
As far as the recommend mods. Thank you! Do you happen to have a link to where I can take a look at them?