Don't be too sure about the 'little slanty' getting too hot, I've had one on my WRX for two years at 20psi with no trouble, and I don't know if you guys think 43degCelsius is hot, but it's pretty warm down here. Never had an an issue with pinging or knocking or soiking EGT's--nothing, the only thing I dont like about it is that it's stock!!! IMPO I would go all out and flip the Inlet around! Relocate your altenator to the LH Side after moving battery etc, then go with full FMIC!!! Dollars allowing of course! I have had a 99 WRX cooler on the top of my 'Beer Fridge' in the Shed for over two years, even got a 99 Inlet Manifold complete aswell, I DEFINITELY would have already fitted them if I thought/new it was needed. but I don't think it is.
Failing the FMIC thing, spend an hour or so looking at Water to air, you DONT HAVE TO use the Subaru Product, there are plenty of Marine heat exchangers that are just as efficient but half the size. Make sure you get a REAL GOOD pump is all....