For any real gains at this point you have to look at getting somthing that lets you alter the fueling.
The site guru Kev does a cheep mod that you can start with, his fuel mod is currently on sale here;
http://www.xt660.com/showthread.php?t=11450&page=2
The more expensive route that allows you to get the most out of your machine is to get something like a power commander that allows you to completely remap your fueling depending on your setup. Kev has collected a lot of maps that members have put together for different setups.
You can find a list of a lot of mods and information here;
http://www.xt660.com/showthread.php?t=630
and if you become a supported of the site you can get access to the Ask Kev section that includes access to the collection of fuel maps other people have produced for the power commander.
http://www.xt660.com/payments.php
Good luck with your mods.