The D300 is basically a repackaged AMD 7850. The 7xxx series cards are 3 years old. Though to be fair, AMD hasn't done much since, LOL; see AMD Fury X
Benchmarks on the 7850 can be found here:
http://www.anandtech.com/show/5625/...-hd-7850-review-rounding-out-southern-islands
3 years ago sounds about right, especially since the Late 2013 Mac Pro you are referring to was released almost two years ago. I don't think assuming a D300 is the same as another card, and using those benchmarks from a windows machine, with completely different drivers, will accurately show performance.
Link to useful Mac Pro benchmarks.
http://www.macnn.com/articles/14/04...s.power.the.mac.pro.to.new.performance.highs/