For one thing, HK does NOT recommend JF mags for use in weapons that might see serious use, such as home defense, CC, or duty. JF mags are only for range and competition use. And for that type of pistol, why not just a JF on a pistol when you want to shoot JF mags since it takes a heck of a lot more time to Dremel down a X-grip than it does to change out a JF. Granted, Dremeling out a XM is a one time thing but on all the pistols I have put JFs on, including one of my USPC .45s, I have yet to change them out since those pistols are for kicks and giggles use only.
By the way, putting a JF on a USPC .45 is way more labor intensive than most would want to muck with. I sure wouldn't do it again. HK45/USPC .45 10R mags are used with modified floor plates since there is no such thing as a JF mag in .45 ACP.