I had a few thoughts about this very subject a few weeks ago.
My thoughts were the reverse of whats been said here tho.
I figure make the lower pipe smaller in diameter than the upper pipe.
This in theory would make the air expand while inside the I/C, thereby cooling itself just by expanding. Compress something, it gets hotter, let it expand again, it cools, right? Do that inside the I/C where that cooling can be more effective, and it might have a small positive effect.
I'm thinking 2inch inlet and 2.5" outlet pipes on the I/C itself with matching pipes.
Of course, this is just some thoughts, but I did run it by a freind of mine who is an engineer(for real, papered and all) and he said it had merit.