I think once the 650 got traction and if the engine is healthy it could run with the 583 but Wildcats are heavy and have zero traction. I had a 550cc (stock) Cougar that would beat 583 Doo's and my buddy had a 650 Wildcat (stock) that was just a tick off my Cougar. I think with equal traction it could be a good race.
Heavy, yes, no traction, I differ on that.
My Wildcat had no trouble getting around even before I changed to a newer track ( 1 inch lug) . even better not and can keep up with anyone I choose to.