I used MediaCoder to convert a 1080p .mkv movie. The original was made with profile L5.1 which was unusable on my Popcornhour NMT.

For conversion to L4.1, I used a special MediaCoder preset intended for the PCH. I didn't adjust any settings, incuding bitrate, which was set at 8755 Kbps.

However, the original filmsize was 15,8Gb and after conversion it was 10,2Gb. The difference must be in video bitrate, since I didn't crop or alter the sound stream.

For future conversions, it would be nice to determine video bitrate beforehand so conversion can be lossless. Is there any way to do this?
Thanks in advance!

I've been converting .mkv files to .m2ts streams with TSmuxer, left at 5.1. It works and the video quality seems to be better than when I did it at 4.1. These have been 720p video streams, not 1080p, though.

For future conversions, it would be nice to determine video bitrate beforehand so conversion can be lossless

If you convert the video, then regardless of the bitrate you use, the video will not be lossless.......unless you use a lossless codec, but then the video will be much bigger than the source material.

Older versions of Super C would tell you bit rates and things if you add the file for conversion then double click on it. It also does a good conversion job too. Latest version doesn't seem to be doing that for me though, but maybe just a slight problem in the software or the version I downloaded. Give it a try though, it's freeware.