I would say that the Dyno-jet kit and the K&N filter combination. You can buy them as a kit for about $110-120. Noticable increase in low end. I don't think that there is any appreciable improvement on the top end. In hindsight, if I had to do over again, I would just have left it stock. There really wasn't anyhing wrong except that I knew I would be taking mine to differnt parts of the country with wide ranges in altitude.
The winch was the first thing that i put on my quad. Never regretted it for a minute.
I said the jet kit / air filter / pipe because you asked for power and performance. The above will give to power for sure. Performance - new tires, the stock tires are crap. As for suggestions there, search the board, since there has been numerous discussions on the subject. All depends on your riding style / ground conditions.
Not sure of your budget, but each of those are going to be in the $300-$500 range.
Bobby! It's yo' daddy.. Roberto! I think a winch is the best "first mod" to get. Get used to the bike first. You may not be a hardcore bushwhacker right now. Learn some trails. Get some riding gear. WHen you get sick of the performance of your Rinny, THEN modify it. Hell, you aint gonna even get that dang thing til next Tuesday, Fartknocker. Learn to ride it before you go ahead and try to modify the piss out of it.
A good reasonably priced mod for the Rincon is Highlifter HD springs. Only about 100 bucks and will help the ride a bit. I agree with rockhead that the best first mod is the winch, either a 2500 or 3000 lb. I really don't think it matters who makes it, they are all good, find the best price and you are set. After you have had it a while, get some tires on it, the stockers give a good ride but are only 2 ply and wear very quickly. Get something with 6ply if you can. Of course it all depends on your riding style. Will you be doing alot of mud riding, trails, staying out of the mud. Once these questions are answered, then you can research the proper tire for your situation. I think the best part of owning a Rincon is doing the research... Just my 2 cents
A K+N filter ($40), Dynojetkit ($40) and a 2" exhaust tip ($35) are great modifications for this machine and it won't break the bank. You will notice the power gain and have a very nice sound without being to loud like some of those exhaust pipes that are outthere. I have this setup on my Rincon and it runs great. I don't have any problems like staling or misfiring and the machine hits 64 mph every time. It also wheelies very easily. The main jet that used was the 140. I have heard that you can get a little more power with the next size up jet(145 I think going by memory)but the machine runs so nice I don't want to mess with it. Hope that helps....