I started out hunting in all metal and toggling to disc and thumbing the disc. Got real tired of that real quick. As far as extra depth an air test will answer that. I have a test garden with pennies, dimes, nickels, and quarters at 4",6",8",10", and 12". The problem is my test garden is still to fresh to get good readings (one year old). I get all coins at 4" in disc. and nothing repeatable after that. In all metal I get some very faintly at the 6" depth, but not with a good enough signal that I would dig. That would seem discouraging, but it takes years for the ground matrix to re-establish so my test garden is not too useful yet. In the field though I hunt with disc just short of zinc penny and I can find coins in the 6-7 inch range. That's not to bad in my mild ground. At depth the signal is weaker but repeatable in both directions. For what it was designed for the Silver uMax is an all around great coin finder with excellent target separation. I would guess in all metal you might get a little deeper, but you will be digging trash. Some people do dig it all and find gold rings and that is what you almost have to do to get gold. I can do it for about half an hour and then it's back to disc. mode. My soil is mild and I can run with sensitivity in the red all the time. The Silver uMax is not a depth demon, but down to 6-7 inches, maybe 8", not much will get by it. For sheer depth you would be better served by the Cibola, Vaquero, or Tejon. For what I do, hunt for silver, the Silver uMax has been a great detector and will always remain in my aresenal. I have one park that I have found over 250 coins in this summer ranging form surface to 7" deep. For more depth I will probably add another detector to the arsenal.
Hunting in all metal to get great depth is a great concept if you have a target ID or multiple tones to aid in your decisions and limit your trash digging. Unfortunately Tesoro doesn't have a detector that fits that bill real well. Hopefully in the future they will. For this I would consider an Xterra 505 or a Whites SST. Mojotrout could chime in on the Xterra.