I don't see any reason the Q125 shouldn't be able to handle it, its a pretty capable esc if you can keep it cool. In fact I think I saw that "some other retailer" was selling the two as a combo for what its worth.
I will ask Serum details about the differences between the Bigmaxximum and 7XL.
I run a 7XL in my supermaxx but I heard great things about the Bigmaxximum: more efficient, smoother and only needs 50-80 amps to produce serious power.
Much of the comments on the Pletty were "overrated". I'm guessing he will say loo at a Neu or LMT if you are looking to go to 4-pole motors. The big brushless dealers here don't seem to carry them anymore (RC-M, starluck or Icare) so that prolly says something.