If you look at my screenshot from MacBench, you can see the results for two frequencies (@16M and @40M; the CPU was always at the default 16M). 40M gives a slight boost to the benchmark score. The card itself has a jumper to configure which clock to use (OSC means the oscillator on the card, SYS is using the C16M signal from the logic board which is 16M).
I am currently working on a derivative of this design that would support the 50 MHz part 68882. It's a PGA, which is the only major difference between it and the rest of the 68882 series. The only reason is "because". Sort of like most of the projects for 68k Macs.