Okay Richard,
As far as I've been able to tell from all the things I've been reading about, USB (3.0, 2.0) are pretty much the same as far as data transfer, and latency issues. Firewire is only slightly better as far as consistency goes, but again the data transfer rate is the same as USB. What makes the difference is how many apps are running in the background. The more that is going on at once in your PC, the more of a load you are putting on your processor and that is where the latency can become an issue. Shutting off all non-essential programs will help with that. In my old desk top PC that I built specifically for recording, I had two separate hardware configurations to choose from at start up, Full Setup, or Recording Setup. In Recording setup, I disabled everything I didn't need, (Sound card, Ethernet card USB Card, etc.) I also disabled all the background programs and apps such as Internet Security and Anti-virus, USB Drivers, Network drivers, Task Manager, etc...everything except PnP drivers since I needed that to allow the processor to see the Core 2 card.
When I recorded through that using Sonic Foundry 'Vegas' or Sound Forge, I had no latency issues at all, and never crashed the computer that way, even recording all four at once. ( The breakout box has 4 in /8 out).
I pretty much was using that in the same way, as a multi track recorder, but there aren't too many outboard effects I would use, mainly the Eventide Harmonizer and one of the Lexicon Reverbs. The ADA gear is fine in my guitar rig and doesn't need to be run with separate I/O's. ( I would need 256 channel strips just to accomodate all the rack effects!)
In any event, here is a link to one of the articles I read, and it talks about recording through USB. This may help solve your apprehensions about the difference between USB 3.0 and 2.0
http://www.sweetwater.com/insync/audio-interface-buying-guide/