I'd like to know to.
But I riot communicate as little as possible on MMR system, to avoid people abusing the system somehow.
I'm pretty sure they don't take KDR in account though, cause that would be stupid. KDR is is no way a mesurement of ingame performance. What about supports ? What if it's just KS ? What if it's useless kills on supports and tanks while the carry destroyed your team ?
More generally, I don't know if there is any way to quantify ingame performance. How do you evaluate the utility of a well placed stun ? Of a shield at the right moment ? Of a nice bait ?