After a few hours of runtime and some heavy benchmarking/stress testing I feel confident in posting my final results.
The cpu seems to average 4.1ghz, leaving mce on. Slightly higher with xmp enabled, but very similar, 4.15ghz.
Cinebench R15 single - 168, multi - 839. With xmp enabled multi jumps up to 874.
Cinebench R20 multi - 2029. Xmp 2116. I didn't run single, as the cb15 score was so close to the 6600. Didn't really see a point.
Timespy - gfx - 11,194 cpu - 4773 score - 9314.
With xmp and a very slight gpu oc, +100 core and +500 mem. Cpu - 5108 gfx - 12009 score - 9985. I'm sure I could finally break 10k with a few more minor gpu tweaks.
Firestrike - gfx - 27601 com - 7361 phy - 12232 score - 18860.
With xmp and the same gpu oc. Gfx - 29164 com - 7889 phy - 12558 score - 19866. Again 20k probably easily with a few tweaks. (Quick edit. Timespy final score with +125/+1000 is 10195, firestrike score 20245!)
My realbench scores jumped an insane amount, but I didn't record the 6600k in here, so I'll skip it.
Onto some gaming. All setting maxed at 2560x1440, no gpu oc or xmp.
Doom 2016. Cpu usage came down quite a bit averaging in the mid 60s with a peak to @88. Gpu and fps stayed the same. No difference in gameplay as it was already as smooth as possible.
Resident evil 2. Again a late drop in gpu usage. Averaging low 50, with peaks in the mid 80s. Gpu usage stayed the same, but it did pick up a little fps, averaging around 125.
Ghost recon. Cpu usage dropped again, with averages in game of 65. Benchmark showed min 45.7, max 81.8, and an average of 57.5. Gpu and fps stayed the same, but gameplay was a whole other story. So much smoother with no stuttering that I saw.
Both far cry titles saw very similar results, with minor bumps in fps and average cpu usage in the mid 60s.
GTA V saw large swings in cpu usage, with only peaks in the 80s, average were much lower, and a slight fps average bump of 10-15. But gameplay benefited most. Smooth as butter with no stuttering at all.
Battlefield. Similar story here. But usage was staying much higher in the 90s, just the gameplay was sooooo much better. I can actually play now, no more twitch or stutter. Stable fps, with averages in the 80s.
Overall very pleased, especially for free. As stated I didn't see any massive fps increase, but gameplay, oh my the gameplay. Better in every way. Games are now smooth, no more lag or stutter. I never felt my system was holding me back. But after these tests, I can see it was slightly cpu bottlenecked, as much as I dislike the term. The upgrade really opened up the system and let it run so much better.