I understand what you mean about getting tired of the Z.
I mean they have great handles, and looks, but power to cost ratio in modding them and etc is not so great. My friend Alex just came out of an integra to a Z, and he's already looking for more out of it. I can wipe the streets with his Z all day from any kind of race.

My friend with a 04 stage 2 sti also spanks on them pretty badly, but honestly I dont know many subie owners who'd prefer the new evo over there cars.

If It were me Id say get a used Evo9 MR or Evo8 and build that.
But thats just my tastes. ( or whichever of those is better, cant remember)
As far as modding the EvoX , talk to BVP about that.
They already have theirs running high 10s, so they can tell you what to expect etc.
They are well engineered cars, with great potiential as the older Evos.

As far as physical space, Id say youd have more room in the EvoX than you will your Z, with back seats and a larger trunk to boot.