I've had one in the past but did not run it in a way like you are considering. I can say for the most part it is a stable detector. The only time I had struggles were around schools or other places that had a lot of Wifi in place. It would not like those areas at all.
However, out in a field somewhere in the country it was very stable. You could run it up near maximum or until it started to false at times. From what I could tell, the stock Fisher coils didn't impress me. When I went to Cors search coils it was noticeable in how much more stable the detector ran. It's still a good machine but there are much better things out there today for the money. I enjoyed my T2LTD (Special Edition) and it found me a LOT of coins with the Cors Shrew coil. Small coil and totally different hunting than what you are considering, but the potential is there for cache hunting. With stock Fisher coil? I don't really think you will get the best out of it. But something like the Cors Strike or Cors Cannon should reach on down.
Even with the little Shrew coil it was no problems to reach down 7 inches on dimes with accurate ID.
