Paula does the analog <-> digital bits, very basic channel en/decoding stuff. The raw data is transferred to a CHIP-RAM buffer where the MFM <-> data stuff is done by Blitter and 68k (Blitter probably doing the sync searching and bit shifting stuff, 68k doing the header/data separation).
If you use 1541 disks in a 5.25" drive (GCR encoding instead of MFM) the Blitter's most certainly no use, so the CPU has to work a bit more.
btw: A CIA does the I/O control of the FDDs (head stepping/selection, r/w switching etc.) - so there's quite a bit of team work involved. ;-)
Edit: Completely forgot about Gary: HE does the write protection, motor on/off controls - even more teaming up.