For several months now, two units of the TRIPP Lite 7+1 Port USB3 hub have been in use on the MPG 2013 Mac Pro and MPG MacBook Pro.
Trouble-free operation with full USB3 speed on all 7 data ports (8th port is a extra high power charging-only port). Highly recommended.
OWC has the TRIPP LITE USB3 hub discounted and in stock.
See also fast USB card readers.