I tested Win8.1SE with 1.2G, 1.5G on VMware, and 4G on physical computer. Maybe that's why I couldn't see FBWF size changed.
I had been hoping FBWF cache size could be set by percent of free ram, like Imdisk.
Maybe there is other solution rather than using FBWF files of 8.0.
If new windows which we can't use FBWF files of 8.0, than we are stuck.