2 x Powercolor Radeon 7970 mounted in a CoolerMaster HAF 925 case with extra 120mm fans inside. 2 for push-pull CPU radiator and 2 blowing on the video card stack from the side. Don't know if an Accelero cooler would perform a great deal better, since the stock ones seem pretty well designed. I may remount the stock ones though. I work at a precision optical manufacturing company and I can use the lapping wheel to grind flat the contact surface to within microns. Could even polish it to within a quarter of a red photon, but I think that would be waste of time, as the thermal paste layer is never close to that thin, and the mirror surface might actually do a good job of reflecting thermal infrared back at the chip. Does anyone here have any experience with diluting silicone based heatsink compound with silicone lubricant or other liquid to reduce viscosity and drastically thin-out the layer? I once did this with GPU heatsink on my Nvidia GTX 570. I got it so well contacted, that you could lift and swing the thing from the bond with no movement. Unfortunately, I forgot to reinstall the aluminum mounting frame before the heatsink, so I had to take it back apart, and the next time around I didn't dilute the compound. So no results to compare.
In your case, the accelero coolers would not be ideal due to the way that they cool. They exhaust the hot air out through all directions except the face of the card where the fans suck air in and of course the PCB which is solid, but all sides and the rear are exhausting air, which would cause hot air from each card to be pulled in as cooling air, so it's not a good solution. The 925 is a decent case for gaming, but for your application, it doesn't move enough air. You could either get water cooling blocks for your 7970s, which would be pretty expensive, but would work nicely, or you could run an open computer, no case, but you'd end up having to clean the computer components a lot more often than with the case in place. Remounting the stock heat sinks would be ideal because it's much lower cost than water cooling and would still allow you to use your case.
I would recommend you clean off all of the existing thermal compound and examine the surface of the heat sink and where it mounts on your GPU/VRMs to ensure they are nice and flat, with no irregularities. Once you have verified this and made both sides of the mating surfaces nice and clean with some IPA (isopropyl alcohol), then apply a nice small amount of high quality TIM to the heat spreaders on the GPU and VRMs and place the heat sinks back on. If you want to be doubly sure, remove the heatsink again and look at the TIM, it should be devoid of any bubbles or clean areas (everything should ideally have a thin layer of TIM on it, little hills or ridges are normal). Clean it off again, as the removal will create irregularities in the TIM (little hills or ridges) and reapply the same amount if it was good, and reattach the heatsink, replace the shroud, and button everything back up.
I highly recommend a high quality TIM like the noctua NT-H1 that comes with the NH-D14 coolers, although you can buy it seperately. People like Arctic Silver AS5, but it doesn't perform as well as several other TIMs, although the overall difference between best and average is only 4 or 5 degrees in the last comparison I read, so if AS5 is the only stuff you can get at a local shop, it's fine. Poor application or too much TIM will make a much bigger difference than which brand you choose. Another factor is cleaning the heat sinks and case to ensure all the airflow is as unrestricted as possible.
As for your performance, it really makes me wish I had a second card, but I'd probably run into heat issues. I'm running mine at 1300 mV, 1230 MHz on the core clock, 1600 on the memory clock 72C 24/7. Never crashes during mining, and I only turn off the miner while I'm gaming (primary purpose of my rig). Hash rate bounces between 715 MH/s and 694 MH/s depending on what else I'm doing, I'd love to nearly double that, but not at the cost of a second 460 dollar card and the possibility of having to ramp them down significantly due to the heat blowing all over the case