even if one has a 64bit filesystem?
There is no 64bit filesystem for the Amiga yet (except SFS version 2.3). These "64 bit" mentioned in TD64 or NSD_TD64 by PFS3, SFS or FFS Version 44+ refer to the commands used to access blocks on the drive. These commands are sent to the device driver (the firmware of the HDD controller). If the device driver does not support these commands, then the file system can still not access the area outside the first 4GB of the drive.
So you need both, a file system using 64 bit commands and a device driver understanding 64 bit commands (more precisely: the same 64 bit commands as there are different command sets).
I don't know if the A2091 supports 64 bit commands and if it does, which command set (TD64 or NSD). If it supports TD64, you can use PFS3 or SFS, if it supports NSD you can use FFS V44+ or SFS. As it is a SCSI controller, it probably also supports SCSI commands. If so, you can also use PFS3ds or FFSTD64 (patch available on Aminet), but not SFS.
If any of the mentioned combinations work, it is no problem to use HDDs up to 2TB with partitions of 100GB and more.
Also the usual patch-combination of a <2GB boot partition which loads NSDPatch to access the other partitions will probably work, but only with Kick 2.0 or higher.
Bye,
Thomas