Find a set of 2011-2012 cases and have them bored to fit the 13+ cylinders with as little clearance between the skirts and the cases. I have them bored so you can barely get the cylinder into the cases. My 2015 Pro has 12 cases on it now, and I have 3 sets of 11/12 cases sitting here for others when they decide to upgrade. I have run 09 cylinders on 600r/800 mono block conversions for years with no issues with the skirts at all (very tight tolerances), one is even an 860 with 09 cylinders. Besides the fact that Polaris is using hot garbage pistons, they are leaving to much clearance for good skirt support in the cases. That along with not setting the oil pump properly from the factory seems to be a common issue. Amazing the difference in stock pistons from one with not enough oil and one that has had the pump turned up.
If the fix it kit that basically lengthens the piston is a bandage so is the long-rod that lengthens to rod, one just costs a whole lot more than the other. They both move the piston up to change the angle and re-leave some of the pressure put on the cylinder walls. Now granted if you want to pay $3000.00 so you get a warranty go for it.
All that being said, I install Fix-it Kits, BMP, RK-TEK's (drop in's), Hanson Racing Engine's, and my own, which are just M8 aftermarket pistons and plate/bolt kit. Honestly think they do help a bit with more case volume on a engine that is already suffering from a case volume deficiency. Plus again using good pistons is a plus compared to stock. Crap I used to install long-rod kits until my supplier started getting threatened legally by another competitor.