I'd stay well clear of the D300's at any rate.
Here's some very informative info for 4K, editing and FirePros
http://fireuser.com/blog/autodesk_is_/amd.com/amd.com/afds/P30/gdconf.com/
The lowest card I see really mentioned with working with 4K is the W7000/D500
With most using the W9000/D700? for real work.
The only time the W5000/D300 is mentioned is for SolidWorks, and even then it's an AMD benchmark in which the card is faster than the NVIDIA K2000. I don't like company benchmarks or tests, as they'll only ever pick what highlights the best of their cards as well.
This is also very enlightening on the performance of Workstation cards.
http://www.tomshardware.com/reviews/best-workstation-graphics-card,3493.html
When it comes to OpenCL Image processing, the W5000 is nearly 3x slower than the W9000
OpenCL Image Rendering the W5000 is around 2x slower on average than the W9000.
Also remember that the D300's have less RAM than their normal Wxxx counter parts as well