I wonder if the GT4 ducktail spoiler (not the big wing) is a direct replacement for the cayman?
If you want to keep your PSE, the only real way to get more volume is by either getting a high flow cat or going catless. If you are considering a tune, I would say just go catless as it will sound the best and the tune will take care of the check engine light. Unless you find a Porsche dealer that is willing to install a tune for you, I think you would definitely run into warranty issues should something go wrong while tuned. Perhaps look into APR as I believe you can purchase a warranty from them (though you would be relying on them to honor any repair costs and not Porsche).