I made no mention of PFS. I'm an SFS supporter! Besides, you could always use FFS and still use the 40GB. Anyway, I now know that 2GB is enough for you, so that's fair enough. However... I have some cdrom images that I like to mount now and again which are crammed full of software / fish disks etc. etc, so this is where your 40GB can come in handy (cd images).
My A500 is strictly KS1.3 with Workbench 1.3.2, because that's what it came with. It has 1MB Chip RAM and the Megabody IDE interface with a 1.4GB Hitachi 2.5" HDD. I am using PFS though, but I might change that to FFS one day. I think I get 700 KB/S out of that. I'd get more speed if I had some Fast RAM. I have one of those Slow RAM cards but I'm not currently using it. I'm toying with the idea of whether to buy a ROM switcher. The thing would have to squeeze under the Megabody though.
700KB/s is a lot compared with the floppy drive speed!