Whatever way works
is the best.
So you've toyed with the firewire chain and have it set that it goes MBP->mBox->LiquidMix on the internal FW400 buss and added an ExpressCard for the drives and it's all good? Then the audio device underflow error has been solved/corrected? You're welcome...moving on to the next issue:
You say you get into sessions with full voice/track count and fairly heavy edits and automation and still your machine bogs down? How much RAM is in your MacBook? If it isn't max'ed out, do it! It's not the drives slowing things down at this point, your processor is choking. It may be time for you to trade up to a MacPro if you're doing that kind of production...or book an HD room for a few hours.
Yes, Higher buffers during mixing help free up processor cycles. Lemme guess: your typical session has at least 12 or 14 reverbs/effects running, and a bunch of heavy plug-ins? there's a way for you to truly have "the-sky-is-the-limit" desktop productions with this rig. You just need to stop and think about how to make it happen...and yes, that's being just as creative (if not more than) as playing/writing/composing/mixing/producing. Doing this at your home "studio", you should have the luxury of time. And if you run into knowledge gaps, do some learnin' up...as you're doing now by coming here. You just need to know the right questions to ask, and how to ask 'em.