So after flashing the F.20, your rabbit is cooked. You get an unmoddable BIOS and in addition it doesn't allow you to go back to an earlier, moddable BIOS. The primary reason for the existance of the F.20 is to lure you into painting yourself into a corner. There are no new features or improvements there, which the high version number would imply. I read in some other forum that the F.20 version is merely a 'trap' for people like us.
If I were you, I'd use the latest modded or moddable version, which appears to be the F.16 version. There seems to be no modded F.18 BIOS here.
