Well, C is never inadequate. Quite a few languages have their interpreters/jit-compilers written in C. C is portable. C will continue to be a major language and will not become some obscure legacy language. You can tell I am a fan boy, but I am a fan boy for a reason. That said, you do not seem to like C, and further you already have the program written. I think you would pitch this as your "pseudo-code" and show it to your customer. Let him or her know that you will be rewriting it and that this could take some time. If your customer is willing to wait, you now have two programs that you can benchmark to compare speed. You can compare development times in a real way, etc... quite an interesting little project once the "project" is finished.
If your speed of development is glacial because the language you are writing in is just a hop skip and a jump away from ASM, then the language you are writing in is inadequate.
This sounds a lot like some random FUD that you picked up somewhere. If C dev times are so long, why is it that new versions of large scale applications written C are available so often? An adept C programmer can get an application up and running in quite short order. Quick GUI? GTKDialog is available as are: Win32 GUI API, GTK, QT, ncurses, FLTK, and many more. It is also worth noting that should your customer/employer be willing to use some opensource code in his/her/their application, the amount of open source C code available online is quite staggering.
