I've used fat boy springs on every Polaris I've owned. When I bought my '04 Pro-X I had the dealer install fat boys before I picked it up. Big mistake. I couldn't compress the rear suspension at all with those springs. I had stock springs back on it before I ever rode it. I'm 6'2" and 225#. Even when I stretched the sled to 136" the stock springs were the right ones. In fact the extension made the rear skid really work well, even with stock springs.
Have you had your shocks rebuilt? Rydes last longer between charges than Foxes but they still need attention. Rebuild Foxes every year and Rydes every other year is the rule we use around here.