Will,
I assume that the stock coil you are referring to is an 8" Coinsearch as the original Sovereign only came with that size coil. Our S-8 (8") or S-5 (5.5") should handle the ground as well as coils of the same or smaller size as they will "see" similar or less ground (S-5). As for the S-12 (12.5") it being a larger coil it will "see" more ground so if the conditions are severe as you describe, you may have to run your sensitivity somewhat lower in order to run a stable detector (or) if in auto sensitivity, the detector may revert to a lower semsitivity to compensate for the ground. If that is the case, the depth advantage may be somewhat less with the larger coil than it would be in milder conditions. However, the ground coverage of the larger coil would still provide an advantage in that respect. How much depth loss would depend on the ground conditions and the amount that the sensitivity might have to be lowered vs the smaller coil. I hope this helps.
Ralph (Sun Ray)