I agree. The DSP from FFE is practically worthless. It doesn't work with most things I've tried. I'd go with picking up a regular DSP game to plug in instead.
It so happens I'm just starting to study the DSP games and may soon have more information on them. One thing I've learned from what I've read plus what I've studied is that one cart will not work in all cases. You need a minimum of two (or you need to hack up a DSP game cart with a switch) because the DSP-1 chip games I've looked at so far (Pilotwings, Mario Kart) use a different address line to select the DSP. You can see a reference to this in DiskDude's SNES Kart doc.