The firmware behaves almost exactly the same. Problem is OSX.
That may be so. Indeed, the problem is often software (or device software, aka "firmware.") But some such problems can be that vendors design and build products according to popular implementation (e.g. Windows USB drivers) rather than specification. For some types of hardware it can get really dicey, like SD cards. An electrical engineer I know was troubleshooting cards and told me in horror of the number of manufacturers that produce cards with characteristics which put the cards outside the SD specs. Its too bad; it means questions such as, "Will card X work with camera Y from manufacturer Z?" really are necessary.
In any case, I am currently weighing the potential risk of having to return a Filco and paying a restocking fee, as well as losing time, against the chances I'll be unhappy with a white tenkeyless others such as iMav have confirmed work with OS X (the PLU-87).
Looks and build quality are quite important to me, but if I had to choose, I'd rather have a keyboard that worked in the pre boot EFI firmware environment than NKRO. Still, I guess I'd rather have better build quality even if it didn't work in the EFI firmware environment.
Maybe its time to try the Filco...