Or unless you misplaced the source code/fired everyone who knew anything about it; which, I suspect, is what Macromedia did with Fontographer.
Did I mention that it (which was last compiled in 1996 or so) doesn't even work under Classic? (It somehow manages to crash Classic whenever a document is created or opened.)
But anyway, re: Carbon. It's basically a translation layer to Cocoa (it creates Objective C-based Cocoa widgets from Macintosh Toolbox data), so Cocoa would be more efficient. Not to mention MUCH nicer to program for.
Sadly, my testing also finds that Fontographer 4.1.5 fails under Classic... Time to break out the IKARUS M, ATF Type Designer, FontStudio et al. who's writing the great Mac OS X type design program? I've got too much on my plate right now...
It's curious to note that the Carbon APIs are not substantially changed since 1983 - so one's Mac UI knowhow is mostly transferable; and no-one has much of an excuse to not rebuild for X (unless your codebase is Pascal 8=).