I say, sit in them first and see if this "shoulder/leg room" thing will become an issue for you, for example if you are on the 'large' end of the population size bell curve. If you are keen to be seen in this car, and if you're going to reward yourself then there's no shame in that, then perhaps the Cayman coupe isn't quite what you'd be after. Unless you're an expert or race driver, then IMO the difference between a Boxster and a Cayman wouldn't be such that it would be a deciding factor. A Boxster is a bit cheaper, too.

HID lights on the Vette, well, they're not essential, are they? I would expect you can get them on the Porsches, too, for a price.

IMO, go for the car that you like the interior of, enough. That's where you'll spend most of your time. The performance of any of those cars should be plenty.