Yes you should be able to overclock the DCUII to TOP speeds. The logic behind getting the TOP right off the bat is that the GPU chips may be higher binned and therefore have passed a higher level of quality assurance. In theory, you can then overclock the TOP, but to a higher rate than the regular DCUII. In theory, the chips that aren't good enough for the TOP models get put in the regular cards. Your mileage may vary, of course.
i got the non top version and i have not needed to OC it at all. Just awesome awesome cards. You know your product is a good one when it comes with 3 separate black corregated boxes with a golden shiny asus logo on each.
As far as i can tell, the top models probably run on a slightly lower voltages therefore have more headroom for overclocking. They are more or less identical and they can all OC to hell.