VMware ESXi compatible RAID card for the Mac Pro
Have you looked this RAID cared benchmark page from 2009? It seems that the HighPoint RocketRAID would be a decent choice but doesn't seem to be available as a new item from the manufacturer any more:


  Not knowing there was a workaround, HighPoint removed the RocketRAID
  4320, 3520, 3510, and 2680 from the Mac Pro product list on their
  website. We're hoping to persuade HighPoint to put them back on the
  product list now that MaxUpgrades has a solution.


A quick check on eBay shows the 4320 RAID card, 3520 RAID card, 3510 RAID card and the 2680 RAID card are still available as a used/refurbished item.

But if buying used/refurbished equipment is not for you, one thing that seems promising and is currently available and in production is the MAXPower RAID mini-SAS 6G-1e1i from Other World Computing. The description seems to be exactly what you are looking for:


  Eliminate the SATA drive bus speed limit! Adding the MAXPower RAID
  mini-SAS 6G-1e1i to your Mac or PC provides a SAS/SATA 6G (6.0Gb/s)
  drive bus with up to twice the data transfer speed, regardless  of
  your computer's built-in SATA 3G (3.0Gb/s) data bus. Utilizing the
  MAXPower's RAID processor, you can maximize your total performance up
  to 4800MB/s in RAID 0 mode or achieve the perfect balance of
  performance and protection in RAID 5 mode.
  
  This feature-rich, high-value mass storage controller card is an ideal
  solution for those working with multiple high-speed HD video streams
  and/or anyone seeking high performance daily IT operation and back-up
  for an SMB. With mini-SAS, there's no loss of throughput as it
  provides full data transfer of four drives through one connection.