The biggest gain I got from the 700 was to remove the air intake and replace it with a big K&N along with a custom intake. Had to use the Dobeck fuel controller to get the mix right but this was worth over 5hp. My slip on exhaust netted 2hp and the race exhaust a couple more depending on the rpm range.
Speaking of RPM, I used the MXU 700 ECU which messed up my dash read out, but gave me another 1000rpm. I don't know how this will work with the 500, might be the same. Changing clutching will help a bit too. Right now I'm maxing out at 56mph and keeping pace with the Arctic Cat 700's. I also did some head work and a cam. Right now I'm playing with a larger throttle body but that has to be matched with a more aggressive cam and even more RPM. I started with 32hp at the rear wheels bone stock and currently have 49hp.