Out of interest, what is meant by improving cmd.exe compatibility?
Add support for all the cmd.exe switches and all the dos batch programming constructs?
I do track bugzilla for any cmd.exe issues regularly and aside from one which I started work on, I don't know of any other batch issues. I spent quite a bit of time working on cmd.exe compatibility, including adding the newer 'odd' syntax (e.g. %~$PATH:i) and getting it as close to native as possible. Additionally, the 'missing' commands are almost all external applications, not inbuilt to the command shell.
I do know of some differences, but few apps require complex batch. You can download some complex batch which require specific text to come out from specific commands, but if no real app needs them, its not worth persuing yet.
Jason