This is a FYI....
Today I spent the whole day trying to sort this issue out for my son who has the new MacPro6,1 (12 core, 1TB PCIe SSD, Dual D700s, 64GB, LG's 21:9 Thunderbolt display, 23" ACD, 46" TV) running Adobe Premiere Pro 2014. The MacPro6,1 system was running the latest Apple Mac OS X 10.9.3 build 13D65.
My son has been plagued by these weird rendered artifacts as described by many in this thread. The issue could be avoided by using software only for rendering, but of course this slows down the workflow significantly. My son has resorted to dealing with 2K projects on his MP5,1 12 core, 2.93 GHz processors running 10.9.3 and Premiere Pro 2014. The render issues have not shown up using this MP5,1 running 10.9.3.
Today I had the opportunity to pursue two options
1) Reinstall 10.9.2 onto the MP6,1 from a full installer image file and then install all of son's software
2) Upgrade a cloned copy of the MP6,1's 10.9.3 Build 13D65 to the 10.9.4 Build 13E25
Since one of the Focus areas for the 10.9.4 is Graphics Drivers I thought this was the best option to try first.
I cloned the MP6,1's 10.9.3 to a spare LaCie 2big RAID-0 test partition and then upgraded this to the 10.9.4 Build 13E25. I then had my son run some tests that always have shown the rendering issues on 10.9.3 to make sure the problem was exposed by his testing... and it was. None of his tests run under the 10.9.4 system showed the rendering issues, and he ran the tests several times just to be sure.
With this result I decided to partition the 1TB PCIe SSD in the MP6,1 into three sections. 1) 300 GB for 10.9.3, 2) 300 GB for 10.9.4 and 3) 400 GB TB for AE_Cache. Before doing this I made a clone of 10.9.3 to a spare partition on a LaCie 2big RAID-0 and used Time Machine to ensure I had current backup of the 10.9.3 system. I then repartitioned the 1TB SSD in the MP6,1. Using CCC I created 'Mavericks' Recovery HD partitions on the two 300 GB SSD partitions for 10.9.3 and 10.9.4. I then used SuperDuper! to Smart copy the saved 10.9.3 and 10.9.4 systems to their respective 300 GB SSD partitions.
I setup to run SuperDuper! to clone 10.9.4 to a spare backup partition each day at around 12:30 am (son leaves his systems running all the time). The idea was that 10.9.4 Build 13E25 was for time being going to serve as his production system. The 10.9.3 would simply act as a fallback if 10.9.4 proved unworthy.
After all this, my son using the 10.9.4 system ran all his Premiere Pro tests and some more things that had caused issues for him over past month or so, and absolutely no issues were encountered.
So for time being I feel that using 10.9.4 Build 13E26 has indeed fixed/resolved the rendering issues encountered by my son. Of course, time will tell if this is so.
When Apple releases 10.9.4 to the general public it will be simply a matter of upgrading the 10.9.3 system to the officially 10.9.4 release. In the meantime son now can use his MP6,1 as originally intended.... that is, to get work done without the losing productivity to the weird rendering issues.