could you recompile a c program in fil-c and then decompile it to c and recompile it in c to have the high performance.
The performance hit comes from the extra work Fil-C does over regular C, so if your decompilation/recompiliation process preserves semantics that extra work (and thus the performance hit) will remain in the final product.
If new code needs to be written, there is no reason to use Fil-C, since better languages with build-in security mechanisms exist.