I haven't really been so amazed by the XM-L. It's true I don't have a 3 A version. I have a 1.4 A version but I just don't need something that's brighter and brighter.
All that really happens is that the batteries just need charging/replacing that much faster.
Whether I have a XP-G or a XM-L I still add a piece of diffusion film and the results is more or less the same. For a larger light a 3A XM-L has advantages but I don't particularly like larger lights.
For a thrower the XR-E still suffices for me and for most everything else the XP-G does it. I have a (2) 18650 MC-E dive light with a 52 mm reflector and an output of 550 lumens or so but other than for diving it doesn't do much for me so I've avoided the temptation to get a larger 1 to 3 emitter XM-L.
Maybe things have slowed down for a while as far as the technology involved is concerned.
The lights I would prefer have more to do with manufacturers being a little more creative or varied than with the need for any more technological improvements to occur.