So...bought a pair of Matrix P's last month, installed them in my trusty P8P67 Pro board, and could not get them to run at over 1100 without artifacting, and even then I had to reduce the memory clocks down to 1500 (6000 in GPU Tweak) or games/benches would rainbow lock in seconds, with Crossfire or without, with one board installed or two, stock CPU clocks or overclocked, with GPU Tweak installed or not, and a clean install of both Windows 7 SP1 and Windows 8.1 on a 840 EVO SSD. PSU is a AX860i, CPU a 2600k (4.4GHz OC), with 8GB Corsair Vengeance Pro 2133 C9, mounted in an Aerocool XPredator ATX-XL case. Drivers used ranged from 13.11 Beta 7 through Beta 9.2, an d the last few WHQL releases as well.
Prior to the 7970's, I was running a pair of shader unlocked 6950's clocked at 900/1300 that didn't give me any trouble.
Yesterday, my Maximus V Extreme and new 3570K arrived (new Battlefield releases always give me an upgrade itch, and the MP's didn't quite scratch it!), so I retired my P8P67 , installed the MVE/i5, and did a "Reset" on Windows 8.1. Did all the usual driver fun, including 13.11 Beta 9.2, installed GPU Tweak, and voila!
Memory is good up to 7000 now at 1.50v! Can get a solid 1275 out of the core at 1.3v! That's in Crossfire, and with the CPU at 4.6Ghz.
Not sure if it was the switch to a fresh CPU over my abused 2600K, better voltage regulation in the MVE, or just the bump up to PCI-e 3.0, but what were cards I was certain I was going to RMA are now overclocking beasts and rock rock rock solid stable and cool at stock clocks. I should mention I'm not using the extra PCI-e power plug found on the MVE, just the dual 8-pin connectors on each GPU, same as it was on the P67 Pro.
Once the shiny wears off my tweaking the new board and chip, I'll plop the 2600k back in and see what the results are...at least that'll narrow it down to either PCI-e 3.0 or an abused, old overclocked CPU.
TL;DR: I no longer blame the Matrix cards for not working as I expected...check your other components, could be one of them is the weak link instead.