From what I have observed over the past several years, there is simply no way to significantly increase a Porsche Flat 6 motor's horsepower output without spending serious money.
Small incremental increases may be had via exhaust / intake / pullies etc. but nothing dramatic. Supercharging is an option but that can get dicey also with longevity.
Aknowles recommendation of a 996 transplant is probably the best approach value wise. Good Luck
