you gotta make sure your power valves are set right and your cables havent pulled through the end of your valve?? that is one thing that will effect some top end performance...
Check out your rear suspension. That is where you will loose most speed, weither being wore out bushings or bad bearings, along with maybe a bad shock.... Are limiter straps tight or any slack in them? Make sure you are running the correct belt..
8400 is where you should be. If your in real loose or deep snow thats all you will see for mph. On hardpack you should see 110ish unless your running a deep lug track or a ton of picks. I doubt you have a pulled thru cable if your hitting 8400 rpm.
Track tension, worn belt, could be alot of things. To help the guys on here help you, tell more about your setup. Alterations from stock on all areas of the sled.