How much horsepower can you gain by puting octane booster in a Honda 300ex?

You will not really gain any H.P. It might make the engine perform better, but that depends what the timing is set at now. Ex: I have a 400 cu. in. Chevy small block, 450 h.p on the dyno, 400 foot lbs. torque. With say 89 octane the timing has to be set at 30 degrees, or the engine will run on when you shut it down. With say 94 octane or better, the timing can be set at max which is 36 degrees. The engine will be quicker and run much better, but in either case, I will still have 450 h.p.