I believe Nexus 10 is still not showing big signs of aging, I mean, the screen is still great, the processor still runs fine everything I need (games like Modern Combat 4, Ravensword Shadowlands, GTA SA [max graphics settings], Cytus and Deemo run perfectly fine with no lag at all!).
The only thing that most people complain and they are right is that due to the huge screen resolution Google allocated 1GB RAM to graphics, which makes it only 1GB RAM. And that means that some apps will close just after you exit them and open another (happens a lot with Chrome tabs too, when I'm with 7-8 tabs open, one or another will need to reload).
I don't know about others tablets yet, I didn't have the chance to check how they perform against Nexus 10. Just don't let benchmark scores scare you, just because one device performs 40k points that doesn't mean it is the best thing ever. Sometimes my Nexus 10 looked more fluid (specially when watching videos) than my S4, while my S4 performed 28-29k points, Nexus performed 24k.
Also there was a video comparing Nexus 10 to Galaxy Note 10.1 2014, due to crappy Samsung software Nexus opened apps faster and was more fluid. The screen had some differences too, Note colors looked more vivid, but that's more of a personal thing.
And for KitKat ROMs, as KK is official it's running perfectly fine. I'm on CM11 after formatting and it was the best thing I ever done to my device. It's blazing fast and does everything I need. And I have no ideas about OC, personally I never used as there was no need.
Just to finish now, Nexus 10 is on the end of Google's 18 month support policy, so if you want to get a Nexus I'd wait for next Nexus 10, as it will have again support. Personally I think that 18 month is now a ridiculous thing, as 2012 Nexis still run perfectly fine, and they can support easily a next update.
"Time is too short to cry, long enough to try." - March of Time (Helloween)
Sent from my KitKat Nexus 10