DWT, I think going to 19/50 will be a big step in the right direction if you haven't warped your sheaves. I think you should bring it to NPS and have it regeared to 19/50, there is a big difference in belt life from 20/49. I have run both, and had way more success with the 19/50. Of the sleds I know of that went to 19/50, (20 to 30) there is a 100% success rate with regard to belt life. You maybe pleased and surprised as to NPS's responce to your request. Good Luck!
The 19/50 gearing is a Pro Climb M sled thing, don't have any experience with XF sleds.
With regard to what I was refering to with the "wave": In under the hood video on 2012 sleds it was revealed that when run at wide open throttle in good shift out conditions the driven clutch would overrun the drive clutch in rotation, "piling" the belt up on the upper leg between the driven and drive clutches causing a "wave" to form in the belt. This is what was causing all the black rubber on the drive sheaves after a belt "exploding" in half. The "wave" causes miss alignment, hooks outside edges out of the sheaves and whamo! This is why Arctic Cat added the plastic shim in the driven, to keep it from shifting out far enough to cause the overrun and subsequent "wave".
Hard to explain without props, but, hope this helps.