i watched a video by Meats Mier for z/maya...what he does is this:

-converts to subd > than create the displace approx and subd approx on your object.

-for the disp approx set the preset to fine view high quality
-for the subd approx set the method to spatial (in here you set the min to about 2 to start and the max to 7....the min is what you edit later until you get the result you want).

(make sure you flip file in zbrush ...the up down button.....and also convert your tif file from greyscale to rgb in Photoshop first)
-now once you attach your file to the displacement node in the SG part of your shader...you take the alpha value taken from zbush and put that in the alpha...for the alpha offset you take your alpha value give it a negative value and divide it by 2...
ex.. 0.44 for alpha would be -0.22 for offset

-give it a test render...probably wont be good at first...so the only thing to increase till you get what u want is the min value in the subd approx.

-also, under global setting for mental ray I set the translation to progress messages to see status....once the render is done, you look at the poly count...once the poly count starts to get close to your poly count in zbrush they will almost be identical

-also, for best results, if your converting to subd and you want exact results as zbrush....in zbrush go to your lowest division, click on the "cage" (it'll look a bit boxy but that ok> export model > in maya convert to subd and itll look exactly like you lowest division in zbrush