The A2091 does ZII DMA so maybe putting some RAM directly on the card, rather then the ZII RAM on the A2058 an on the A2620, might speed up diskaccess a little bit.
Maybe you will have to reduce the amount of RAM on the A2058 after installing RAM on the A2091, as the total max of ZII RAM is as mentioned 8 MB.
But I wouldn`t invest in such an old pice of hardware. You should really get a faster accelerator tha usually has a way better SCSI-controller. A Blizzard2040 would be a good start and not too expensive. If it is cheap and there is already a decent amount of RAM on the card (it`s practically imposible to get the unique GVP RAMs today, unless you are wiling to pay a fortune) a GVP030 would do also. Stay away from the Apollo2030 as it is totally crappy.