when a car is using a stock or even modified ecu, starlets have a fuel map that as the rpm rise, gets richer and richer. so , if when you boost lets say 3500rpm you have 12.5 AFR and at 6700rpm you have 10.3, you cannot remedy this by leaning out the fuelling by lowering the fuel pressure, you will get a lean spot in the lower revs. So, either get a piggyback if you dont like being so rich in the upper rpm range, or a full standalone ecu. But by all means if you don't know what you re doing , take it to a professional for tuning.
or buy a book and do it yourself, save yourself the money (you will have to read it , though)