The first question is how large is your codebase? If you have a large system then there will be a lot of code to translate to another language, which itself will create an overhead.
The other thing to think about is that there is very good interoperability between Nim and C, so you can always look for a C programmer that may be looking at running a mixed language project. Use Nim where he is comfortable and use C (or C++) where is is more comfortable doing that.
