So given that you are dealing with a rear engine car then maybe the water/air is a better route. Here's the thing, on the intercooler itself, water to air is very efficient, the problem is heat soak. At some point, the water will be hotter than ambient air, and will lose it's ability to remove heat and will become ineffective. Because you are rear engined however, using a decent sized radiator just for the intercooler water, you may be able to beat this trend, or at least slow it down. It's just alot more components involved to make it work. Pump, hoses, radiator,fan for radiator,intercooler,etc... and if you ever have an intercooler rupture, you have water straight into your motor. (happened to me in a marine application) I think it's the best route given your size restraints and probably the amount of air you can actually get through a rear mounted air to air cooler.