At ~3k RPM the gain is 11 HP, which works out to over $400 per HP.
Most engines live at 3k more than they do at 6k.
You want a Boxster with grunt, buy one with a blown engine and ship it to Jake, pay for one of his performance rebuilds and get the added displacement that really helps throughout the entire RPM range. He can get you to over 300HP if your pocketbook is big enough.
You want more sound for cheap, how about doing the Pedro's Sport Exhaust mod
link .