I would say this would require a few more questions/answers and some testing. But based on my experience will give you an answer. First I know ground conditions will definitely have an effect. Second there are different small coils, of the two brought up the most>5.3 and 4 x 6, I believe the 5.3 is slightly deeper, but the 4 x 6 has better separation. Third with the proper adjustments on the dfx I know for sure depth can be gained. I know someone will bring up a few other smaller coils but those 2 are talked about the most.
Now my answer to the question, I have hit dimes for some odd reason with the dfx using the 5.3 Eclipse coil easily from 5 to 7 inches. I notice in soft sandy soil or wet soil from wet I gain at least an inch or two! I usually can up my pre amp gain from 2 to 3 with no problem, on the dfx 4 is pushing it sometimes but have heard people using 4. remember different detectors different users, settings, knowledge, soil conditions, all play a role.