I raced a 650 sn in the vintage class up here on the greatlakes watercross tour. A few mods that are essential, tubbies, shortened handle pole (I had a RN pole), intake grate, rideplate. Once the hull is all squared away (pun intended) I would proceed to drop a 701 in and start to mod that. The vintage class here had all sorts of skis so the 650 if ridden correctly was actually relatively competitive. Ofcourse a large course on calmwater the 650 will get smoked by the 750's and 701's, but you get in rough water or on a tight course the 650 will hold its own assuming you can ride it.