You could go with a disc type muffler system on the end of your exhaust pipes. Alot of race cars use them to meet sound regulations at various tracks. More discs = less restrictive (more power)... less discs= more restrictive (less power).

Keep in mind the fewer disks you use the more backpressure you are adding and that will affect torque as well...

pull a touch of timing out?

Put some sort of plate between the throttle body and intake that will restrict airflow slightly and tailor the opening to get the power you want? (like a restrictor plate in nascar....)

http://www.supertrappind.com/Automot...otive_page.htm
http://www.supertrappind.com/Automot...%202002-10.pdf