I don't have it, but some thoughts... I would use it for backup with the 17 rounder, but something I'd be checking out is, how much, if any, does it move and is there any chance it would keep the mag from fully seating at that worst moment.
In other words, if the extender (a spacer) isn't seated fully against the mag floor, when you put it in the mag well, the X-Grip might prevent the mag from seating right away and cost you precious time or worse.
So I'd be testing that and if that's the case, I'd find a way to prevent it from slipping.
A note about longer mags used without a spacer... if under stress you slam it in pretty hard, it's possible to over insert it... and cause a fail to feed or worse. A spacer stops the over insertion... you don't want to depend on the mag release catch thingy to stop it from over inserting.
If the X-Grip doesn't fit so well, just trim it a hair...