Your right, there is a lot of power to be had. The 88 HP RZR 900 is the same engine, but the upper end is different. Everyone is focusing on power improvements for the RZRs. Theres not a lot of money to be made by the aftermarket companies on the Rangers, most go fast guys buy RZRs.The easy way out is to buy an RZR top end, but as ghostrrider6 mentioned, a reprogrammed RZR ECU is going to be the tough part. The pin connectors between an RZR and Ranger are different so you cannot just switch them. A programmer from someone like Power Commander wouldbe nice.
I went through all this with my 09 Ranger when the 800 was the big dog. I spent an incredible amount of money on it.