Well don't expect much from an A2091 maybe 1-1.5 MB/s. And in a ZIII machine you absolutely must have some RAM in ZII adresspace (best you have the two MB on the A2091 directly) as the A2091 does do DMA, but only in 16 bit. If there is no RAM in the 16bit ZII adresspace the controller do PIO-transfer -> some 100kb/s.
See also
http://www.amiga-hardware.com/showhardware.cgi?HARDID=1161
