Two things ive done with sucess in the past that worked and was very cheap was-
A) Use a pressure switch (oil pressure is fine, in the boost pipe) to switch a relay that flips between coolant temp resistance and a variable resistor. So YOU can adjust the mixture on boost (set it richer by increasing the resistance and fooling the ecu into thinking the car is cold)
B) Use same pressure switch to join an aditional injector so that it is pulsed along with the cars original (run the earth frrom the fifth injector to the switch, then to the cars original injector (control) earth)
Both work, the first is more adjustable, but offers less overal increase. Maybe a combination of both would work? And could be done for pennys too!