The short answer to your question is this (from FF website)
"All FFI Power Steering boxes are thoroughly cleaned and inspected, the cases are machined for roller bearing sector supports and the worm gear assemblies are resized, the box is then fitted with Firm Feel's exclusive reaction springs"

The typical parts store rebuilds don't do the machining, resizing etc. So IMHO the best reason for getting a FF box is actually the first part of that sentence - that you get a properly rebuilt box.
The second bit (the firmness/reaction spring etc) is also something I want, but I'd rather have a stock-effort box that is tight than a sloppy firm box (actually I want both but you get my point)

If you have a box that is in great shape, no slop etc, then do the washer thing and enjoy!