I had the 525 and it was nice in the dunes. Then I installed the factory 540 kit with the 8/06 cam, and head work. It was absolutely by far the nicest "I" had ever riden in the dunes. It had gobs of torque plenty of hp and would pull the hills like a mad man. I would still get out run by some of the highly modded 450's and 700's. Now I'm having the engine built to a 600cc by Thumper Racing with the lightened balanced crank shaft, 8/07 cam, head work, and he is also boring the carb to 43mm. When it is finished it will be putting out torque #'s somewhere in the 50's and hp in the 60's. So it should be a screamer when it is finished. Some of the people on here are getting their 505's built to dyno in at 60hp but there torque #'s are still in the mid to upper 30's I believe. With that being said the stock 505 will rev farther and faster than the stock 525 with a about 7 more hp but about the same amount of torque. The 505 is limited to minor builds like higher compression pistons, after market cams, and head work. So the question is how much do you want to mod the quad. RFS is much easier to mod and the #'s you can get are pretty amazing. Stock for stock I think the 505 would be hard for anything out there to beat. But once you start moding the engines, well, so far the 505 is very limited. There is no replacement for displacement. I hope this helps. Good Luck!!!
Brian