Fastest SSD in Mac Pro 2006 thru 2012, Leopard and above
PCIe-SSD which surpass SATA III speeds can mean a lot, and for a 1,1 is the only way to boot from PCIe (whether SATA III or not).
http://www.macintouch.com/readerreports/harddrives/index.html#d17feb2015
Samsung XP941 256GB PCIe 2.0 x4 M.2 SSD MZHPU256HCGL
M.2 Interface: PCIe Gen2 5Gb/s, up to 4 lanes
512MB LPDDR2 DRAM Buffer Memory
Support TRIM Command
Sequential Read: 1080MB/s, Sequential Write: 800 MB/s,
Random Read (QD=32): 120K IOPS, Random Write (QD=32): 60K IOPS
Works with (all) Mac Pro. Not compatible with the MacBook Air or Retina MacBook Pro
http://www.amazon.com/Samsung-XP941-256GB-PCIe-MZHPU256HCGL/dp/B00J9V53M6/
A smaller XP941 128GB that 'only' gets 450MB/sec writes instead of the 800-900MB/sec
http://www.amazon.com/NGFF-PCI-Express-SATA-Adapter/dp/B00M8HC5JC/
Lycom DT-120 M.2 PCIe to PCIe 3.0 x4 Adapter (Support M.2 PCIe 2280, 2260, 2242)
http://www.amazon.com/dp/B00MYCQP38/
http://barefeats.com/hard183.html
SATA Express meets the ( '09 ) MacPro - Bootable NGFF PCIE SSD
http://forums.macrumors.com/showthread.php?t=1685821
http://www.engadget.com/2015/01/08/samsung-sm941-pcie-ssd/
Next generation from Samsung:
Mac Pro, Mac OS X (10.7.5), ATI 5770 16GB Samsung SSD Sonnet 6G