While I'm not familiar with the CMI, the BMS micro racks -- available with long and short frames, single and dual hyperbars -- are very, very popular among cavers in a variety of settings. I would venture to say that the BMS long-frame, single-hyperbar micro rack is the most popular micro rack among cavers right now; many cavers leave their long-frame racks at home for anything other than deep, free pits. In a general sense, micro-racks are typically more suitable for caving use than a piranha, due to the factors Scott mentioned: stainless friction surfaces will last tremendously longer on gritty rope, and a micro-rack won't tangle the rope as badly.
However, I'd have to caution against using 8mm rope for caving use*, at least anything other than pull-down trips. This is especially true for common canyoneering ropes like Bluewater Canyon Rope (or Canyon Pro) with a particularly soft hand and superstatic core. The exception would be for those who are absolute expert-level riggers and plan to always rig in an "alpine" style with multiple rebelays and deviations to prevent the rope from ever touching the rock. One of the fundamental differences between caving and canyoneering is that we usually end up ascending the fixed ropes we rappelled in on, and I am constantly surprised at how easily "small-diameter" ropes (and ropes with a supple sheath) will fray or cut against limestone while climbing.
The small diameters and polyester sheaths that make for very nice canyoneering ropes often make for very, very bad caving ropes.
* The only 8mm nylon/nylon caving ropes I'm aware of are made by Cancord and Beal, and even Beal hedges their bets by referring to the 8mm Antipodes, anecdotally, as "accessory cord"
here.
"Although it pains me to say it, in this case Jeff is right. Plan accordingly." --Andy Armstrong