I'm with VitaminC, I think he gave you your answer right there. You're not going to find much more storage in the same generation 996, as they're the same from the doors forward as a Boxster, and clearly are lacking a rear trunk for obvious reasons. While you may end up with more space in the back of a C5 Z06, I doubt it's by much when you total up both the Boxster's trunks. You may just have to get creative and sometimes selective with what you do store in your trunks. I noticed you didn't mention golf clubs, so I'm not going to bother making any points on that, as your idea of sports seems to be more along the lines of actual activity. (No offense to any avid golfers)
None of the three cars really screams the utuility you're looking to achieve, so I would say in that aspect, enjoy what you have, not that you'll be trying to flick the car around bends with a kayak up top (at least I hope not).
Now regarding power, from the prices you're likely to pay to get a 996 or Z06, I would imagine you should be able to get yourself an engine swap done for nearly the same, if not less, in the end. Others with more experience or knowledge in the matter would have to confirm/deny this, but I can't imagine you'd be paying $24k in the end to swap to something like a 3.4. Try getting in touch with Jake Raby of Flat 6 Innovations. He's right across the street from you in Georgia, and that's his bread and butter. :dance: