you're in a pretty big crowd of folks who want a faster winplc.
the major issue is that it will take a complete redesign to make it possible. the hitachi processor we used (sh1) topped out speed wise at 100mhz, and then microsoft's wince support essentially dried up except for a few other processors.
changing processors, along with it's companion chips, is a huge undertaking. we'd need significant marketing interest to start something that major, and so far, that interest just hasn't been there.