Might want to take a look at Dev's speedfarming build from the last patch. It's slightly out of date [in terms of the info] but 2.3 didn't change all that much really.

I didn't think about Falter stacking; I just round robin it. It might, though.
The various runes of war cry do stack.
Conventional wisdom is the gem stacks; unsure about strongarm.

More generally; you have to wonder if doing higher level greater rifts with less +xp would be faster than doing lower greater rifts with more +xp. I dunno; but it's an interesting idea. I'd recommend building towards 4 speedfarming builds, and then just gimp one or two [ie toss on xp gear] and see how it goes.