you may want to download GPU-Z and see what the usage is like (gpu load %). if you remove all of the gpu fx and get smooth playback, that might also suggest the gpu is holding things back. if you decide to upgrade the gpu, you should look at the new gtx 1060 6gb. its faster and has more memory.
what kind of storage drives and setup do you have?
Thanks for the quick reply RoninEdits, it's very helpful. Media is on an internal 2 TB RAID1, don't think that's it. A quick test confirms that clips that stutter with multiple layers of color correction do not stutter with cc disabled. I'll confirm with GPU-Z, but I think my suspicion is confirmed. Do you think the 1060 is compatible with my system? I know the power supply is large enough.
the pcie gen 3.0 video cards are suppose to be backwards compatible, but every once in a while an old motherboard and new gpu don't get along. sometimes a bios update on the motherboard can fix it, or a different brand gpu. you may want to search the web for your motherboard and any of the new gtx 1060/1070/1080 cards, to see if people are having problems. i searched for a few minutes and wasn't finding anything. i found some using the gtx 900 series, which is a good sign the gtx 1000 series might work. the i7-3930k cpu you have is only pcie gen 2.0, so it won't be able to run gen 3.0 speeds. that may slow down the gtx 1060 a little, but not enough to be concerned about.
if you can borrow a new gtx 1000 series card from someone to test, it could also confirm the upgrade is needed and those cards will work with your motherboard. buying one from a store that has an easy/good return policy might also be good if there is a problem.
(overclocked by ADK to 4.5)
I'd give Eric at ADK a call.