I doubt anyone has benchmarked how much "CPU" the ADAT "uses" versus the 8 analogue outs (and I doubt you'd notice any difference in the Akai's behaviour - I've never ever heard its audio output glitch due to CPU issues)
As for the difference in sound quality of different sample rates and bit depths -
come on

you know the higher ones are better. However if you can't hear any difference, you won't care.
(Your MPC4000 loses half its polyphony and on board FX at 96k - but the onboard FX suck anyways).
Basically, any sampling I've done on my own MPC, I've done it at 96/24.
But as you've noticed, all the existing sample libraries I own at at 44.1 (or even 32!)