Yes, imperial stouts will need a larger mashtun than the 5 gallon water cooler to do 5 gallon batches.
I use a 5 gallon water cooler MLT and I can fit 15 lbs of grain MAX (16 if I wanted to stir mud). I've done two 15 lb batches and both have worked out very well. I can get about 1.07-1.1:1 water:grist ratio - I don't really know because when I'm done adding all grain I bring the liquid level to within 1/2" of the top. I find that the 5 gallon cooler works well for nearly every beer I'd want to make with the exceptions of RIS, barleywine, and probably scotch ale. I should mention that I batch sparge and generally won't mash out unless my grainbill is in the 8-11 lb range.