"Let me make sure I understand, you think if this place does your motor it will reduce the value?
http://www.tpcracing.us/
Are you serious?"
It all depends on the buyer. Personally, I would not buy any modded Boxster so I would to me, it would reduce the value.
To others, perhaps you, that would not be the case.
On balance, once rarely if ever, regains the value of these types of mods. It happens I suppose, but I have never seen or heard of it.