AFAIK UAE maps the Motorola FPU instructions to the corresponding Intel processor instructions which have 64bit precision at most and nobody has ever noticed any problem with any Amiga FPU software. 64bit of precision is still an awful lot. I think it is safe to assume that the lost 16 bits of precision will not make any difference at all.
Huh, no! The legacy intel stack-based FPU is 80 bit, AFAIK.
If you use the SSE (vector) unit, then that's 64 bit. It really depends on the application what you need to do.
I think this model is not too bad. Have a legacy 80 bit FPU if you need to use it, and a 64 bit fast vector unit if you have applictions that require it.
Certainly, that's more work - for little benefit.