Well, this is totally against my expiriences. If you write a clean program in C#, its farily faster than Java. Those converters are often total crap, I used recently one which converts Java Bytecode to Native code. Resources usage was nearly the same. Those automatic converters aren't optimized for speed, they are optimized for accuracy of conversion. I get eye cancer if I see such converted source :/ That applies btw even for the VisualBasic to C# and reverse converters.
As I (hopefully) ported the plugin API to work with C# (found a tool which generates reasonable code, desipte from way too long variable names), I can run some speed tests with a sample plugin.
Edit: Oh, and by the way: if this works, it would be a optional plugin, so you can use it, if you want. You don't have to.