no clue about the volcano, never read much reviews, but then again I'm not really up on the latest air solutions and whatnot. I know with my amount of case fans, AS5, SLK-900A combined with a vantec tornado I set the bar pretty high as far as cooling goes.....the bigwater might do better now that it's getting hot and basically the fan is only pushing hot air lol.
As for adding a GPU , yeah you can but you'd need to make sure to get 1/4" tubing......also I can link you to a place where you can get a $25 pump to increase some performance because you'd really want to get a little more flow and pressure through there adding a GPU block to it.
I honestly don't know how much better my dangerden will make temps on the CPU and it'll be considerably better than the bigwater, so as I said in my case I set the bar kind of high for good temps. You WILL notice a nice decrease in temps on the GPU though that's for sure. That's mainly what I'm looking for when I get my set all built.
Regarding that pump I gotta ask my friend where that was at, as I forgot where it is currently lol