Personally I dont get why everyone seems so obsessed with size (thickness) when it comes to the cooler....
Its something that interested me when I first started tuning Subaru's.
What about the design? Tube and Fin v Bar and plate etc.
And more importantly its overal design, particularly the fin design.
The function of the FMIC is not to be large in size and therefor flow more air, I doubt that even the oe coolers are restrictive as far as airflow goes. The turbo will have a max air flow and thats likely to be the most restrictive part as far as mechanical airflow goes.
Its all about its ability to reduce charge temps, allowing us to maximize performance. Large surface area is obviously good, assuming it can gain free ambient airflow across it.
I started typing some points up but actually this is a very good source of info.
http://www.are.com.au/feat/techtalk/intecoolersMR.htm