Personally i think it's cheaper to sell the 2.5 and buy a 3.2.
And if you really want more power there are some V6 and V8 options at the same or much higher costs than a 3.2. Don't forget that you have to adapt and add a lot of parts to put a 3.2 in.
And if you want to stay with a Porsche engine and want to have much more power, than this is the way to go – but for the money you again can buy a much newer car with lots of hp:
Regards, Markus