well the good news is you can indeed already just have unified cpu/gpu memory on linux with an igpu on good old replaceable ram. i've done it on 8th gen intel stuff i picked up dirt cheap used. for the most part, running on the gpu wasn't faster (nor appreciably slower) than the cpu for the things i was doing, just more power efficient. overall bandwidth is relatively limited regardless which would be the bigger difference comparing against the m chips. and of course, good luck if you're hoping stuff like opencl support hasn't long ago been ripped out of whatever software you might perfectly reasonably expect to run this way today