I'd agree the h110i gt is a great cooler and a custom cooler will be better but not massively. It would cost a lot aswell, few hundred dollars more for several degrees. Best to keep it simple as it will have a big effect on the gpus and cool them a massive amount over stock 40c rather than 80+ on standard cooling, gives you loads of headroom.
Look at the corsair 900d super tower. Bigger than the current full tower case you have. And has a bottom side support for a 480mm radiator. Giving you loads of room to fit the resivoir and pipe straight through the bottom keeping it nice and neat.