To the command that kdenlive generates "acodec=aac ar..." add
preset=slow. I'm not sure if kdenlive honors that but these presets are available for x264 in ffmpeg and avconf. They are good presets and are the same that you could choose from when you recorded your video (
superfast in your case).
Slow is generally a good option if you want to avoid visual quality loss.
If adding that line didn't work you can try to generate a script instead and add
-preset slow after
-vcodec but before the output file.
You probably want to delete the
vb=%bitrate% bit, the preset will have its own value for that.
If you want to be really lossless in terms of video information you need to choose a lossless codec. I'm a bit puzzled that you have so few formats available. Maybe the "Show all profiles" checkbox gives you more options?
A recommendable codec for screen recordings would be QuickTime Animation or any other RLE based codec.
The Dirac codec from BBC is also a great lossless opensource codec, it can be found under the name
libschroedinger in ffmpeg.