The top mount intercooler (TMIC) which sits under the scoop on the top of the engine on all these cars can be susceptible to under-bonnet heat, especially following being stationary after heavy driving. For this reason it is sensible to allow the "heatsoak" to dissipate with good airflow at low boost after fast road driving or if you have had to sit stationary for a while.
The core area exposed to airflow and the volume and mass of the core have increased as the years progressed. The Samco or similar intercooler hoses available for the UK MY97-00 models have superior strength and airflow and are a very worthwhile modification.
The effectiveness of the OE intercoolers varies throughout the years. The early 93-96 slanty type will limit power to around 310 bhp whilst the 99-00 can run to around 330 bhp. The STI MY01 onwards is capable of over 450bhp with an appropriately sized turbo.
Fitting a later type intercooler is a popular upgrade although it is by no means a simple task. Some custom fabrication can be required in most cases. There are a number of after market top mount intercoolers which are true bolt on items which are capable of high 300 bhp.
The front mounted intercooler (FMIC) is an effective upgrade, particulary on the MY93-96 cars as it keeps the intercooler in a cool area reducing heat soak and ensures an adequate cool air flow at all speeds, the downside of fitting a FMIC before it is really necessary is the increased lag after a gearshift and sometimes it induces compressor surge.
Fitting complexity depends upon the manufacturer, in most cases cutting of the bumper is required. Off the shelf units are available to support up to 800 bhp.