[BScout]

A 12 core 7,1 vs a dual-processor 5,1 isn't really a boost in power (you can check the numbers.) The 5,1 dual-processors still hold their own compared to many later Mac Pros. (The other features though: noise, power consumption, heat production, faster bus speeds like thunderbolt, might be a more important concern for you.)

HDX cards work fine in the 7,1
Depending on which slots you use for the cards, you *might* need a different power cable. The top ones don't need it. The bottom ones do need it.
I have been able to put together a 6 card HDX system together in a 7,1 though (and a project used 5 of those cards for DSP)