Yeah ive always wanted to do that. I was thinking of building a tiny PC with a 240 rad in to cool the CPU and GPUs on stock settings or slightly undervolted, so i could take wit with me when im working away etc. Then when i got home plug a massive external into 2 quick release connectors on the back and crank up all the voltages and settings.
When i used to run my 290x crossfire setup i could undervolt/clock it so it was only pulling a few hundred watts which the 240 rad i have easily handled the CPU and gpus when undervolted, i tested them out when i had the loop in bits once.
That would be perfectly fine as it would still max out everything at 1080p and my monitor i used to take with me when working away was a 1080. Then when i get home and plug it back into the 1440p and crank the settings it would start drawing 800 watts again so then the big rad would keep it cooled.
Thats one of the complaints i have with these 980tis, i cant figure out how to get them to underclock far enough, because when im browsing and watching videos its just burning power for no reason, with my 290 setup i just hit my underclock profile macro button which locked the gpus and CPU down to min settings and virtually idle power draw.