I know you didn't ask me specifically, but I'm one of the few to have experience with the K10, 8C, and the older 8.A CIEMs. When I got the 8.A I was a huge fan of it. Then came the 8C and it became my favorite CIEM. Now the K10 seems to be even better. I was concerned because I tend to like a slightly warm tilt to my speakers or headphones, and the 8C seemed to be exactly that. See my review in my profile for further thoughts.
I need to spend more time with the K10 but so far it's brilliant. Very slightly warm and smooth compared to what I'd consider purely neutral. The 8C has a more pronounced note thickness to it - sort of a similar presentation but taken a bit further than K10. That may or may not be a good thing depending on your preferences. K10 does win on technicalities regardless of signature, but 8C is no slouch for sure.
In short, the extra money buys you more subtlety, microdetail, more genre bandwidth (some might think 8C is too warm and smooth for certain music), and improved soundstage/imaging. It may or may not be worth it to you.