A site that can be used to compare intercooler size flow. Not perfrect but informative.
www.bellintercoolers.com
Go to core inventory
click on 3" thick core
Part number 3002400600 is a 3" thick core with 6" tubes and flow entering on the 24" side and exiting on the other 24" side flows 770cfm
Part number 300068240 is a 3" thick core with 24" long tubes 6.8" high
flow entering the 3"X6.8" side flow 171cfm.
All flows are done in the same manor. Different manufacturers measure flow with differing pressure drops. I go here with core size to use it as a standard.
Your car will need more than 171cfm to make the power you are making. Therefore there is most likely a pressure drop accross the intercooler.
If you lower the back pressure by reducing the amount of pressure drop accross the intercooler you can pick up power.
Could you use water injection and not intercooler, maybe. Some people do it.
On my SAAB in installed a 489cfm intercooler to replace the 100cfm intercooler. Exhaust manifold pressure dropped 5psi- power went up. The fuel mixture droped from a 12.5 to 14.0 because there was less spent cumbustion left in the cylinder.
So many things to consider and balance.