When upgrading intercoolers you run the risk of a few side effects, however the difference between and STI and OEM WRX is not that much so you really will not have to worry..
The general side effects of upgrading the TMIC to a larger one on a stock or too small of a turbo..
1. Increased spool up due to larger "Plenum" space that you now need to pressurize.
2. More air volume which could mess with AFRs..
3. Colder intake air temperatures which will result in yet more air volume furthering effects of #2..
Did I forget anything??
For this application as I said prior the STI is not much bigger so your not going to get much of gain or side effect..
02 WRX.. "It's fun when it runs.. Problem is that it hardly ever does..."