Is this really a suitable counter-question? GPUs lend themselvse towards specific kinds of programming problems + added latency of GPU communication. On the contrary, the cost between switching from any on-CPU language is programmer time that may result in significant runtime advantages.

Also disadvantages: increased programmer time for implementing new features. Risks: you might optimize the wrong part (i.e. you invest into rewrite, but it will not solve performance problems).

That's why you must quantify advantages and disadvantages, including risks minimization and only then you'll see, whether given course of action is viable.

You should probably first consider seeing if there are critical bits of code that can be rewritten to use MMX/SSE instructions, since your data is in the right cache already, without needing to move it anywhere else.