My Gp32 also actually only 'supports' that 156Mhz (tested with almost all emulators/games for Gp32 so far; above that (even 160Mhz) it freezes after 1-2 secs).
But amazingly with GpCinema all MHz values work. Ok, I only tried movies with framerates below 16fps.
Could it be that GpCinema is that clever enough not to use my choosen 166Mhz with movies below a certain fps?