Was just wondering what's the current state of 802.11ac on older Mac Pro's with it's inclusion in the latest MP? Is this something we can upgrade via a new airport chip?
doubtful.
1. May or may not be the same vendor Apple is using now. So no reason to think any custom card would be multiple vendor friendly.
2. even if match vendors the card's power requirements are likely now different from 2008-2009 specs. ( chips most certainly on different level of fabrication process level. )
3. AC requires MIMO (mult in , mult out ) antennas to go faster... just do not have multiple antenna infrastructure in old Mac Pro designs. 802.11ac drove the Airport base stations to an entirely new case. Mac Pro doesn't need 6 antennas for beam forming, but 3 would likely be minimal (e.g., iMac ) if actually want to get to the higher speeds.