I'm not sure. I also don't know if the inflation would be strictly a MKV:M2TS ratio, and I don't know if audio/video tracks have different impacts on the end size.

I've fought with it before, just going from m4v exported from premiere, and then muxing to m2ts. Thought I had calculated everything correctly, but M2TS files are just bloated. I think all containers add a bit, but MKV fares much better.

I couldn't find any solid info on calculating the overhead, so I just did it empirically and, after a few wasted encodings, ended up with an approximate ratio for the project.