Am I right in thinking I can swap my '99 classic ECU with a v5/6 STi ECU for a slightly higher fuel cut? Its a striaght swap right, just plug and play?
Any other benefits from doing this?

 

In a nutshell no you can plug and play as the crank and cam sensors are the opposite way round on the JDM ecu.

Also the JDM ecu is mapped for a completely different spec and runs much more aggressive timing. Best option would be to stick with what you have and get it mapped.

Banny