A custom block is probably your best option, most of them perform better anyway...and will work out cheaper...
EVGA allow card cooling modifications as long as you do it right, any damage caused as a result of improper installation will be at your own cost, that seems fair to me though.. The rest of your warranty is unaffected, just keep your original cooler as it needs to be re attatched before sending back if anything does go wrong
Ya, I just did an RMA this week, and part of EVGA's instructions was to put everything back to it's original state before sending it back, though they did mention the sticker has to remain on the card as well. (It's on the PCB not on the cooler).
Their RMA process has seemed good so far. They approved me the day I described my problem to them on Saturday, and after them getting the card, they have sent back a new one a day after they received it (probably refurbished, but they did not say). The UPS tracking system has it coming back to me on Monday, exactly 1 week after I sent it out on a 3 day delivery.