Looking at the path of the water in the block I think it'll fare a little worse that dedicated block, but with the fans and heatsink, it'll more than make up for it. However, one of the points of a water block is to do away with the fans, for a silent gpu. I wouldn't dare run that card without fans, that block won't cope, imo.
Having said that, with a bit of modding you can achieve the same yourself
I wonder if Tom will tear this card down? Be interesting to see the construction, and also the performance of course.
Are dual method cooling graphic cards a thing of the future? The Poseidon is a good start, but welcome other manufacturers to try harder!