It's possible I don't know what I'm looking at, but my Fenix PD10 Ti has an XP-G R5 in it (unless they're lying to me) and it's about 4mm square, including the PCB it's mounted on -- in other words, the same size as an XP-E and much smaller and thinner than the older LED the Arc6 uses.
Certainly the Arc6 could be retrofitted with an XP-G, but for it to be just right it would need a new reflector, and maybe a backing shim, depending on whether the reflector was designed to reach into the head far enough.
On a personal note, my preference is for the larger-format LEDs. I don't much like these new tiny ones -- they're too hard to work with by hand, though I guess they might be easier for flow-soldering machines to work with.