I am guessing your talking about XDCAM HD.
Can’t answer the upgrade to 5.1 question
You can transfer full XDCAM HD quality over firewire because it is a transfer process and it’s twice realtime.
If you are mixing HDV footage I would shoot in the SP 25mbps record mode with the XDCAM HD which is the same as HDV and there is no mix and match issues.
We are using our AJA LA with Studio 2 with no problems. It didn’t require new drivers, the latest for the LA work fine. Io Driver V3 for Io/IoLA/IoLD (Release Date 4/4/2006). We are on a G5 Dual 2 Gig. OSX 10.4.9. I would call AJA Tech Support.
It works great in FCP, but it will analyze your hole clip. So make sure the clip in the timeline is not attached to a 30 minute master clip. The render on a 5 second clip was less than a minute.