Currently testing my 7970's in Windows before I flash the BIOS to the settings I want (voltage, clocks, for maximum performance) and switch over to mining in Ubuntu, and I'm testing with AMD's 2.7 APP SDK. When I'm mining with just one or two cards, I hit 99% GPU usage and I get all the performance out my 7970's; when I'm mining with five cards, GPU usage wildly fluctuates between 70-86% and I net substantially lower hashrates. What gives? Less GPU utilization equates to lower hash rates! I'm suspecting that either the SDK or mining in Windows itself is to blame.

Specs:
5x 7970's: 2 Diamonds, 3 Sapphires
Windows 7 Ultimate x64
AMD 11.12 Drivers + AMD APP SDK 2.7
cgminer: process priority high, CPU usage between 6-15%, poclbm kernel, 2 threads, intensity 10, powertune 20, mining with EclipseMC

• Any suggestions?
• Any evidence to the contrary that SDK 2.7 works just fine? Is it just me having the issue?
• Do the Linux and Windows SDK's have the same performance, ie. if SDK 2.7 is netting 70-86% GPU usage in Windows, it will be the same in Linux?
• What is the best SDK to use with 7970's in Linux? In Windows?