From what I've read by people whose opinions I trust, using a RAID system for sample libraries won't give you any speed advantage. In fact, it might be best if you dispensed with the RAID system and used individual FW800 drives (good) or eSATA drives (best). Being that you're using a MPB, though, I'm not sure what options you have for eSATA.
That said, computers are notoriously flaky for playback of live virtual tracks. And Logic can sometimes be a bit flaky in the playback department, where one day a project will play back fine, but the next day it will spit out "disk too slow" error messages (stemming from playback of virtual instruments). So my emphatic advice would be to print stems for playback during the show.
In fact, there are no advantages to running virtual (MIDI) tracks during a live show over running audio unless you plan to, say, sweep a filter of a synth part live or something along those lines. You're not going to be tweaking (say) the velocity of parts while the show is going on. Playback of audio tracks is so much less CPU intensive that you'll find safety and stability in playback of audio tracks.
So the steps to take would be:
1) run your virtual tracks during production rehearsals and work with the FOH to adjust the balances, EQ, and so on. Once you've settled on a mix and satisfied with playback of the parts, print stems. To that effect...
2) Converse with the FOH mixer as to what he/she needs most in terms of flexibility without the number of stems becoming overwhelming. For example, it might be absolutely sufficient for FOH to have simple stereo stems of (say) strings, woodwinds, brass, percussion, and so on. They might want all solo instruments (regardless of the orchestral family) to be routed out of a single output, or particularly boomy percussion (gran cassa, bass drum) to come out of another.
The rule for live playback is "keep it simple", in all respects, including...
3) Your computer. It should be a dedicated computer that has no access to the internet (Airport turned off), no active Widgets, nothing that will sap computing power. Energy Saver prefs should be set to [i]never[/i] let the computer or hard drives sleep, and the screen should not be allowed to sleep either.
Finally, you should absolutely get yourself a good UPS (uninterruptible power supply) and run your computer and all hard drives from that at all times. Surge protectors are definitely insufficient because they don't provide backup power. A great place to get UPS's is www.refurbups.com. Go with something that outputs a pure sine wave, like the various APC "Smart UPS Tower" series.
As far as humanizing parts that are too difficult to play, my sincere advice is to get players who can play them. You can spend inordinate amounts of time trying to humanize quantized MIDI data and still not have it sound good. Human comes from humans! :-)