The issue with an MR2 is it's so much lighter that even a 300 shot would allow it to keep up with those cars. Maybe a 400 but none the less. A friend and I have devised a way to test your theory. Let's see if this works.
2 honda fits. Automatic, stock trans, stock suspension.
1- all power upgrades, no weight reduction. No N2O.
2- all power upgrades, no weight, hp at 50% so 1/2 the power and a 100% shot which would double the HP, so they should run even.
100% shot- nos fit pulls by 5 cars.
70% shot- nos pulls by 2 cars and looses 1/4 in 4th gear, nos fit wins by 1 3/4 cars.
55% shot- nos pulls until 4th gear where all motor pulls back to win by a car.
Next test-
148 hp fit aka 1/2 my hp(not many parts, no weight, nos 100%)
My fit still the same as last test no nos.
100% shot- pulls 5 cars.
CONCLUSION- 100% shot is approx 250.