It's all about turbo compressor effeciency.
You dont want to go over board on the intercooler because bigger doesnt mean lower ACT's but it shure means more lag and loosing boost.

Check what compressor you have on the turbo. Check the flowchart and with some math you can know if your compressor is up to the job or not. If you get anything near the 65% island, then consider to hybrid the turbo to a bigger compressor.
Only after getting the turbo to suit your engine's tune you choose the smallest but up to the job intercooler.

I've seen too much ppl going for big GRS's intercooler when fitting a esc cossie std one would make the ACT's almost the same.

No need to waste money and boost for lag.