Have to keep in mind that once you increase speed then power will suffer. There is always a trade off there. These machines are designed to be work horses and hill climbers and to crawl thru deep snow and mud. So if your going for more speed then some of these other features will suffer. There are after market kits to allow the rev limiter to go higher but I don't see that being a smart idea as the engine will suffer and loose life quickly. Another option is to go for bigger tires that is a larger diameter. Here you can gain top speed but will suffer in low end grunt, which will cause excess wear on the belt. There are also kits to change the ratio that the sheave clutches operate that can help top end and some kits are designed for more low end grunt, you can go either way.
Just keep in mind the trade off, power vs. speed and also the time, effort and money required to make the change.....will it really be worth it? Realistically now....if you want speed then trade your machine in for one that was actually designed for speed, some of these performace machines will exceed 65 mph. Not real sure I would want to speed thru a trail at that speed and clip a tree!! I am not a fan of spending bucks for repairs and broken bones...shortens up my riding time to much!! I have the 450i version and my limiter goes off between 48-50mph depending on conditions. But then ....not interested in letting my engine crank at 7500 rpm for any period of time so these bursts of speed are short lived. I typically cruise around at 30-35mph on the road (allowed to ride roads in my area) and never really find myself more than 20-25 on trails at best.