I am using the CalDigit FASTA-6GU3 here as well and it is working fine in my 5,1 Mac Pro. I have the eSATA ports connected to my RAID-5 backup system and a Voyager Drive Dock. One of the USB3 ports connects to my RAW STEEL memory card reader, and I use the other port for portable disk drives occasionally. I am seeing no delays or sleep issues with the eSATA ports here.
There is another long thread here discussing this card along with others that you may wish to find and read. The CalDigit does require a USB3 driver (not using the native OS X driver), and apparently can't achieve maximum speed with 2 ports active due to sharing of the controller (not really an issue for my application).
Another popular card are the USB3 only (no eSATA) Orico cards which do use the native drivers, however the 4 port card requires additional power cable to function.
I needed a single slot solution for both USB3 and eSATA, thus my use of the CalDigit card.
-howard