Drop the weight, it's a physics thing. It will improve acceleration, braking and cornering (faster timed laps). 5-10hp will help acceleration only and you will still get walked by the S2000s on the straights because they have 40hp more, and shorter gears so they put the power down more effectively on long straights. Weight is free and won't require you to punch a hole in your firewall. Drop the weight and get them in the corners.
Another tactic is to run smaller diameter tires to effectively lower your final ratio and shorten your gears.