I've done lots of mods so far, but none performance really.
in terms of styling The best way to update the look of the Boxster or to make it look like a far more expensive car is to Upgrade the Wheels to 18" Carrera 5 spokes or Turbo Twists (more of an agressive 911-like look) and add side skirting. This will cost about $3,000 but make it look like $15,000 more.
here's one with only a wheel upgrade and it looks great.