I've had the EVGA block and tossed it - too restrictive, and to open up the block to clean it/look inside you have to tear off the sticker. Also, tearing off the sticker makes it look even crappier haha It's much more restrictive than the other blocks, without any added performance. Losing scenario.
The EVGA backplate works with the Koolance 580 block perfectly from everything I have seen. I was about to order one, but switched last moment to the Heatkiller - beautiful block IMO. (this one:
Watercool Shop) The 'hole' edition.
However, the EVGA backplate doesn't exactly fit with this block. I have made mine fit, but it took a lot of cutting and extra hole drilling to get it on there. So you can do it if you're resourceful, but for a straight out-of-the-box fit with the backplate go for the Koolance.
Also, one note about the HK block -> if you use 3/4" OD tubing, don't expect to be able to use a straight compression fitting on the block without an adaptor. You have to use a 45degree angle adaptor because of the spacing between the inlet and outlet ports. In this manner, the Koolance is a little better for ease of use with fittings as well as it being a bit lighter in weight than the Heatkiller block.
Overall, I love the HK block though. Very solid. IMO much more solid than the Koolance will be, but the Koolance block is a little more forgiving with fittings, weight and backplate fitting.
Here's a snap of the EVGA backplate on my Heatkiller block... not exactly perfect by any stretch, and I could've done a bit better job but it's on there none the less.