The 3D that you get back will be calibrated correctly in the sense that in enhanced mode, it will ID in the ground, older nickels, Indian Head pennies and some small gold coins into the high tone category. The premise of the enhanced mode function is to use it in older areas where there is less aluminum trash. It can be used in that mode in more trashy areas, but will ID some pull tab pieces, small can slaw and other aluminum with a high tone ( which is where coins come in at). What you can do however, when hunting trashier areas in enhanced mode, is when a high tone target is hit, you can switch over to normal mode to see if it now hits as mid tone. This is a method to help eliminate some aluminum trash being dug.
IDing the older coins with a high tone allows one to coin hunt and concentrate on the high tones, leaving the mid and low tones in the ground due them being more of a probability of being trash (but as we all know, until you dig it up, you really don't know).
As far as depth goes, a Tom D calibrated 3D should ID a clad dime in an air test in the 11.8-12.2 inch range. This is with the stock 8 inch coil and the sensitivity in the 4.5-4.8 range. Only caveat with this test is, I do believe these numbers are for the older Los Banos tuned 3D's ( ones starting with the 1021 serial number and some with another number which I cannot remember...this is why you hear about the 1021 3D's being mentioned as hot ones). Apparently the newer 3D's are now tuned correctly as the procedure to tune them was changed, but some of the newer batches prior to this were not tuned the proper way and may have not been optimal in the depth department. I may be totally incorrect on this, but I think this is what I read on a post by Dave J awhile back, as he was trying to explain the myth behind only 1021 3D's being tuned correctly.
If yours is a newer unit, I am not sure how much more depth Tom D can squeeze out, but he will always re-calibrate to make sure it ID's the way it should.
In any event, you will have a really good all around unit to hunt with when it comes back from Tom D.