would by handy a "--turbo" switch that will disable/lower some values (subme/ref/...)
so users will be using a standard "slow" profile in two pass mode, BUT simply adding "--turbo" (and not removing other options) to first pass string , will got a huge first pass speed boost (as xvid option)
about preset: you surely know x264 better (and a lot!) then me, but you could take a look at HDConvertToX or MeGui profiles
in hdc i have build profiles based on your suggestions
last one: +1 for crf 21
BHH