Hmm, lots to think about(still)
1-A single 192 can do 16 inputs at 96K. Adding an input card to a "standard" 192 covers that need(unless you have something that can connect to the AES inputs).
2-While some other hardware options may be considered "better" sounding, the 192 is totally capable of making great recordings.
3-Its nearly impossible to add Thunderbolt to a computer that wasn't built for it.
4-With an HDN card(which is what I would use with your Mac Pro), you will be able to get up to 64 IO as the max, with digilink interfaces connected(giving you future expansion options)
5-AFAIK, other DAW's can use the HDN card/digilink IO's(I know many folks that run both PT and Logic on the same system)
Hopefully, this fills in a few more blanks
Re 96K vs 48K, here's my 2 cents; Does 96K sound better? In a direct comparison, sure it does. Does it make better music? Nope
Having recorded and mixed projects at 48K that got Grammy nominations and ICMA awards, I believe the gear has less to do with success than many think. Mostly, its down to the skill and taste of the artist(s), the engineer and the producer. The gear takes a back seat. Give me a killer performance of a great song done on an "average" system over a crap performance of a crappy song done in a million dollar studio
I'm not saying great gear doesn't matter, just that its not going to make you better than you are. Aretha Franklin singing into an SM58 is miles better than me singing into a vintage U47
