Firewire is faster than USB.
Burst speed on USB is fast like you say at 400, but doesn't hold up under sustained transfers..like audio/video.
Latency might be better on Firewire.. but is something to be dealt with in PTLE anyway.
I mean "bounce" or re-record your tracks to another track, or render it so you aren't using so many VIs and plugins. This would free up your processing power.. but of course you will have to make a decision about what the sound will be.. as you will not be able to adjust it again without going...
You mentioned you are doing post audio work.. If you are doing any film/video audio work, PT is a must! Digi paved the way in the early 90s.... something young people don't understand and old ones here forget.
The price to run a PT system has never been cheaper!!!!!
Oh, and by the way...