Here are few thoughts I've had over the years.
Capacity is important, but consistency reigns supreme. ICs are about providing a consistent charge of air to the engine, no matter what the boost is. The air temp that exits from the end of the IC doesn't have to always be the same as ambient (the closer the better), but you do want it to be cooled to ambient, or above, following a consistent ramp of temp vs. boost.
Optimize the internal surface area for pressure drop and the external surface area for cooling capacity, but above all keep that IC temp consistent, and you'll have something works about as well as it can.
When I had my top mount and put the car on the dyno, the TMIC became so hot you couldn't touch it. Then I had an FMIC and an even larger turbo, even after several pulls it was at most slightly warm. The TMIC does not offer a consistent cooling effect and that limits your tuning options---you could have an "interheater" afterall. Better safe than sorry, and that means tuning for the most heat soak possible.
We could all probably get by with top mounts or at least V-mounts if we never sat in traffic, but for a little more pressure drop and a little more intake volume, I get a more consistent charge that allows me to tune the engine to a limit that I could not if I occasionally had an interheater instead of a intercooler.