At least you can buy the 6990's. I don't see the problem with the warranty. Those 7/8 more than likely won't cover a card if you kill it from overclocking either. It's the same as the older GTR's launch control switch. You use it, warranty void.
EVGA/ASUS also don't warranty 590's either if you adjust the voltage out of spec.
Also, the sweclockers 590 was only at.. "GPU VCore @ 1,025 V". It's obvious right now that any voltage change in the 590 will kill it. Which is kind of pathetic that the power regulation is running right at the very limit. Nvidia says that the max voltage you can add is 25mv... Which is pretty much nothing.
Keep digging Rollo, you might hit solid ground eventually.