Tokyo Motorsport in Welshpool have a pretty good reputation - Known for their work on later model Evo's but have had my VR4 in with them for a clutch change and timing/balance shaft belt replacement and was happy with the service.
Garage 101 have also done some tuning work for me (haltech) with positive results.
Are you running a standalone or factory ECU?
Here's a list of Mitsubishi specialists in WA with AWD dyno: