There are many aftermarket ECU options available from numerous suppliers. We have found the most capable and cost effective unit for MY93-00 to be the APEXI Power FC.
We have achieved up to 800bhp whilst retaining factory quality idle and excellent low speed driveability.

Unlike many aftermarket electronics products, the Power FC is not an interceptor and isn't packed full of useless gimmicks. It doesn't take the signal coming from the airflow meter or MAP sensor and alter it, it doesn't change output signals from the stock ECU as they head towards the ignition coil switching module; it doesn't have flashing LEDs and quaint slogans, No need to add an ugly flashing light knock sensor to the dash as the check engine light can be programmed to flash if knock is detected. This would normally only happen if you get a bad batch of fuel.
The Power FC is a complete, drop-in replacement ECU - once it is fitted, the stock engine management ECU can be literally removed and placed on the shelf. Even better, the APEXi unit plugs straight into the standard loom, and uses all of the standard output actuators (injectors, idle speed control motors, etc) and standard input sensors.

The following maps are able to be altered:

20x20 Ignition timing and Fuel correction maps (thats 2x400 different map points), Injector Opening Time and delay, Airflow Meter Type, Individual Injector Trim on all cylinders (reduces risk of No3 running lean), Boost Pressure, Acceleration Enrichment, Cranking Enrichment, Coolant Temp Injection Correction, Rev Limit and Idle Speed.

Full boost control is available within the ECU up to the limit of the cars OE map sensor. An additional sensor may be added for running higher boost pressures, this can be either an Apexi 3 bar map sensor  or a 97 onwards map sensor ( MY93-96 require this map sensor upgrade to run above 1.1 bar boost controlled from the ECU, 97- cars have an increased limit of 1.8bar as std)

We have fitted many of these to performance enhanced Impreza's. The results speak for themselves, up to 350 bhp with full supporting mods on the standard td05 series turbo with improved engine safety levels. Engines with custom build and upgraded turbos have made over 800 bhp whilst retaining excellent driveability and economy when off boost.

Wiring loom modification, ECU installation and full custom mapping using wideband Lamda and electronic knock monitoring is included in the price.

Mapping is also available throughout the UK and Europe at an additional cost of £200 for the 1st day. 2 Cars can be mapped per day.  Any 'group buys' will attract further discounts and travelling costs of £80 per subsequent day. Typically splitting the travelling cost down to only £70 each car if there are 4 cars being mapped over 2 days. 

Any further mapping to optimise the engine to new components subsequently fitted is available at a fraction of the initial cost.

Apexi Power FC  and Commander supplied, fitted and custom mapped £795
 Mapping of a customers own ECU is £280