Well, I finally got around to setting up my OWC ThunderBay IV this evening. Currently have 3 drives in it. A 512GB Crucial M4 drive, and 2 4TB and 2TB WD Black drives. I'll buy a fourth 4TB HDD for an internal Time Machine backup later this week.
Since I'm coming from a MP 3,1 with its SATA II limitation, I expected to see increased performance on the SSD compared with my internal bays on the old MP. My new setup is a late 2013 rMBP model (11,3) running a 27" NEC PA271W on the other TB channel, so I'm not sure if these are sharing an internal TB connection. This might affect things compared to a nMP.
My Blackmagic numbers:
1. 512GB Crucial M4:
MP 3.1 internal - 240.8 Write / 267.3 Read
rMBP via TBay IV - 248.6 Write / 385.4 Read
2. 4TB WD Black (about 55% full):
MP 3.1 internal - 141.9 Write / 140.6 Read
rMBP via TBay IV - 140.6 Write / 142.2 Read
So it looks like the M4 performance is much better on Read and a bit better on Write, while the TB connection on the HDD (which never maxed out the old MP) is about the same.
A few other observations.
1. My assortment of drives all mounted without problem, so does not appear as fussy as the Pegasus2.
2. The fan is loud enough to be noticed, at least when on my desk.
3. I also tried sleeping and restarting a few times, and the drives seem to reappear fine.
4. The drive caddies appear to go in and out smoothly .
5. When I open Disk utilities in 10.9.1, the S.M.A.R.T. status is available.