I was just thinking about this for a coin I am considering making... I think it's a good idea for the following reasons.

* Primecoin started out very low performance and the community took it upon themselves to improve it.
* If the coin is a really good idea, people will adapt your c# to cython/c/c++ to gain an advantage and after the advantage is leaving they would open source it

I think this would basically add 1 extra step to the arms race that happens with new coins. I.e, Bytecode, machine code, GPU, ASICs, etc.