Hi Matt, Yes I have.
My GTX780Ti is from Asus, there normal version with radial fan and not over clocked. My runtimes are around 24000 seconds with Win7. It seems that with XP or Linux the runtimes are between 17000 and 19000 seconds. I have tried a lot of setting with Asus GPU Tweak the bundled software. Advice in some threads was to down clock a little, but that seems not to work. Windows WDDM is hampering the card.
For the rest the card is running error free since I have it from October. Only one error, but that was because I needed to reboot the system as it was frozen after updating GPU-Z (graphics card monitoring program for Windows).
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Greetings from TJ

The following thread had a few discussions on slow times for the 780Ti: http://www.gpugrid.net/forum_thread.php?id=3583

My specific problem was using the integrated Intel GPU for other projects which resulted in the cards being slowed down 21% on Santi WU's. Disabled using the iGPU to avoid the penalty. http://www.gpugrid.net/forum_thread.php?id=3583&nowrap=true#35027

As TJ noted below, (and various threads have discussed), the Win7 performance hit to the cards is quite a bit. There is an interesting thread discussing a ".config" (not cc_config or app_config) file which collatz project is using. http://www.gpugrid.net/forum_thread.php?id=3626 More details are at http://boinc.thesonntags.com/collatz/forum_thread.php?id=1009&postid=16503#16503