This is only applicable if you have a SW20 gen 1 or 2 (not a gen 3 or gen 4).
The MR2 computer will run based on the information from the Airflow meter and Throttle Position Sensor TPS. If either are incorrectly set by the slightest it will impact the air/fuel mix and performance.
Airflow Meter
http://www.mr2oc.co.uk/know-your-2-knowledge-base/mr2-mk2.html?kbartid=270 Ensure you look for your airflow meter setting stamp (mixture stamp). Pictures are on the how to guide. Most likely it will be hard to see the stamp, use some fine smooth sand paper and cleaning product to see it (mine was very hard to see).
TPS
Second thing to do is ensure your TPS is correctly set. I cant find the easy link, so best to go to the BGB link
http://staff.osuosl.org/~bkero/bgb/mechanical/index_mechanical.htm , select Volume 1, sub heading EFI, then Air Induction, Throttle Body (3SGTE). Read down the page for all the values it needs to be set at (easiest if you have someone help you, and dont loosen the screws too much only enough to move the TPS as when you find the spot/setting point you dont want it to move).
After all the above is done. Warm the car and take little notice of what it does. Then clear the ECU fault codes, disconnect the battery or main fuse in the engine bay, wait ten minutes. Reconnect and start the car. Take it for a few drives, taking it easy, and you should find a better performing vehicle (cleaner cold start, better idle, better open throttle) than when the settings are out.
Everything you need to know about your MR2 mechanically turn to the BGB
http://staff.osuosl.org/~bkero/bgb/mechanical/index_mechanical.htm And the MR2 forum :)
Off for a club run, all the best,