With CGXMode you can set this higher, but higher rates not supported officially. The limitation is in CGXMmode not the drivers. Some have reported that you can run at 100mhz in 16bit without problems. Yes the ViRGE can do planar 2 chunky conversion too. The roxxler can do p2c slightly faster.
Nothing has been planned so far, things are pretty open still.
This is why the card has such a strange shape, to allow this. No, it is quite the same display engine. This are only examples though, in certain cases more is possible.