As to your original post - I think your getting a decent deal here, I know your taking a big hit, but year one is the biggest one, your losing over £10k in VAT alone, so you have to expect a £15k hit in year one in my view, or the OPC is making the money somewhere else?
As for the difference between a GTS and an S? - I don't think there is enough of a difference that you would notice it enough daily to really care about it if you buy an S - on a track a GTS may be half a length quicker than an S, but on a normal everyday road it's irrelevant, it's all in your mind, but that can be a big influence on it's own to some?
Spec wise I tent to look for things I want, rather than things I don't. So if I really wanted something that was not on the list I would miss it, but if I got extra things I didn't care for then it would be no big deal. The pano is one of those I can take or leave, and do think people may want it, but they will not pay an extra £2k second hand for it, so I would never spec one at that money, I'd rather have the extra head room and lower weight and COG.
As previously mentioned, this is your money and your choice, it's hard to find two cars specked exactly the same in any Macan, so it shows we are all very different, and spec our cars accordingly.
The only other thing I'd add would be I'd look at how long you intend to keep this car and have a think about the resales then - so if your going to keep it 3 years, think three years from now and how much will you get back on the GTS Vs the S, then work out the saving your making now and add them to those figures to get as best an estimate as possible about the total depreciation over those three years of the two cars - that would probably be my deciding factor as I view the cars as very close, and both a good buy to begin with.
Good luck with whatever you choose, I'm sure you will enjoy either, and I also believe your doing the right thing, because what is the point in feeling you have missed something when you can afford to sort it with some extra cash you won't miss? If you would miss it you would not be looking at a new Porsche after 12 months!
James