Every card is going to react different. Plenty of FE cards can hit the same clocks as the classifieds, and plenty of classifieds are limited to much lower clocks. This is the silicon lottery, and it isn't always perfect. Classified cards are tested the exact same way reference cards are to verify they work at their specified clocks. Unfortunately, that does not guarantee that you are going to get perfect overclocks.
For voltages, you should get a probe it cable, and test the true voltages with a multimeter rather than looking at the software. Software has never been quite right, and may never be able to properly read the voltage.