Top mount intercoolers on road courses.
 

A lot of people who run road courses don't increase the size of their top mount intercoolers because they tend to become interheaters in these situations. I have found that with any size intercooler that WI always performed well in managing intake temps on road courses. This would seem like even more of a reason to stay small on intercoolers with WI given the decreased pressure drop. This would also be a reason to keep the IC top mounted - the routing is much shorter and the pressure drop and lag is a lot less. With water injection controlling the temps everything should be good. I really need to try it without an intercooler at all sometime. :)

Are you proposing using the stock top mount? The biggest reason to change intercooler for road racing when using water injection is to reduce pressure drop. The larger the core opening the more air it will flow with the least amount of pressure drop. The lower the pressure drop the less exhaust manifold back pressure the more power made for a given boost.
If you are using something like ECUTEK you can davance igntion timing in off boost transition to case the engine to acclerater quicker.
I would refer you to www.bellintercoolers.com to compair flowsof different size and configurations of cores. Lowering the exhaust manifold backpressure for the same boost pressure is more power.
The short distance from turbo to intake valve is a big conderation. I would suggest Subarus top mount was more a design to prevent damage in WRC events and the like.
A good test for your current setup would be to logg intercooler discharge temps for an event and see if the intercooler is getting heat soaked.

I haven't seen any evidence that the pressure drop from the intercooler is lower on aftermarket designs than the OEM. Most people upgrade to larger cores because of improved intercooler efficiency which should be less of an issue with water injection.

The WRC subarus are front mount - but they also have a reversed intake manifold which shortens the return routing significantly. They use the scoop for the air intake box directly down into the turbo. I think the production subarus are more of a cost issue with the top mount.

When truly testing intercooler effinciecy temperature drop would need to be compared, pressure drop over the intercooler as well as exhaust manifold back pressure. The continued reference to exhaust manifold pressure seems annoying but it is important. Never thinking about it until it was compared to before and after an intercooler change.
A lot of upraded intercooler do not aways flow more air, they may cool better but with the same or increase back pressure.