Adapters go from $9 to $54, the latter having heat-sink which is not needed in an open cMP.
Some support both as in either or but not two at the same time, SATA interface on the PCIe afapter, or an M.2 type
While other PCIe "solutions" exist they are not OS independent or machine, the XP941 suffers from none of those concerns as it is bootable in all Mac Pro 1,1 thru 5,1, and is the only one for 200-7 models (and 2008 are iffy) so you can 'take it with you' if you move to a new cMP.
Old controller, you need two SSDs in RAID0 to get 900MB/sec writes.
Does not appear to be any of the real issues of PCIe slot and bus master, outside of you would exclude the 1x slot in the 2006 of course!
The hardest part? getting these tiny M.2 blades, which are ~ 1" x 3.25" and have a tiny screw, on the PCIe adapter.
250GB SSD today are ~$120 + many do want SATA III PCIe (to get 550MB/sec and allow continued use of 4 x SATA II drive bays) runs ~$80-299. In some cases you then need a 2nd SSD. So if you want speed (vs size) then $240 + $100 and the $300 for XP941 256GB isn't bad.
Finally, look for an Apple blade on ebay if running Yosemite - they vary, some are 2x (slow) M.2s and you want 4x but any are probably 700MB/sec and cost $400-700. No need with Apple part to worry about TRIM.
Comparing 128GB SATA II $83 + $15 adapter vs PCIe XP941 128GB $140 + $24 is a 'toss-up' in cost but not performance, and one frees up drive bay and uses PCIe slot. (Generally 250GB are faster and better, the 128GB use to have 135MB/sec writes and $40 extras for 550MB and double storage makes sense, you never want an SSD to get down to 20% free, and you want to have 11% set aside for over-provisioning).
So in a way you can get 1000MB/sec like nMP for $175-300 in any Mac Pro, and new faster blades are on the horizon offering 1400MB/sec.
Kind of amazing, that M.2 support is in that old EFI all along waiting 😉
