Minelabs don't air test well, but in a low EMI environment away from anything my GT will air test pretty darn good. On the other hand, I've dug coins deeper than these air tests show with some of these coils...
http://youtu.be/Ym__l_JtJtU
I've posted some other videos on that channel dealing with various settings and the notch and all that jazz.
In or near a home, be prepared for instability without lowering sensitivity a ton. I would recommend using Auto sensitivity to learn how the machine is supposed to act when stable on your first few hunts before switching to manual. Most of us use zero disc and notch, noise band 2 to match all the older Sovereign charts of prior models, and Iron Mask ON for a bit more depth and better ability to pull coins out of iron. What can be confusing is the way the Disc Iron Mask ON/OF/All metal switch interacts with the PP/All Metal Fixed/All Metal Track switch. Simply put, most leave the one in Iron Mask ON, and the other in pin point (PP) mode, so when you flip the disc switch to all metal it defaults to PP mode and not one of the other all metal modes. Many people though don't even use PP to pin point targets and instead do it in disc. If you aren't used to a DD try using the tip and wiggle forward until you just hear the target to find it right at the center outside edge of the coil, but in disc it's easy enough to use the center of the coil too.
A nice slow sweep up to maybe medium speed (depending on your soil) is what these Minelabs want. But when you happen upon a target, especially a super deep one, you need to wiggle over it in short wiggles or short sweeps and keep that in constant motion to pull the best ID and tone out of the target. Don't stop, keep at it to work up the ID/tone to it's best # and pitch. This isn't unique to the Sovereign. My QXT was the same way. When you do this "Sovereign wiggle" the machine seems to go into an enhanced amped mode of some kind to try to pull the best ID out of a deep one. I can even hear a sort of "click" and white nose effect to my GT's audio when it does this, and then a click out of it when I stop. I still wonder if this machine had maybe an aftermarket mod done to it (Dixie mod?) that I didn't know about when I bought it used, because I haven't really heard anybody else mention this click/white noise thing. Anybody?
As for the 15x12, very stable coil like the 12x10. The 15x12 will greatly help faster gridding of large open fields or beaches, but in my mineralized soil I suspect it sucked up too much ground matrix, because it didn't get the depth of the 10" Tornado on coin/ring sized targets. It did go deeper than the stock 10" Tornado in my mineralized sand though for some reason on coin/ring sized targets. The 12x10 for me is deeper on land than either of them. If you look in the coils sticky thread at the top of the forum you can find thread links on various coils. The stock 10" Tornado is one excellent coil. Best stock coil I ever owned on a machine, just think the 12x10 takes it up a notch in all respects for me. For a trash coil look into the 8" (7.25") Tornado as it's got an excellent reputation for that and also as an everyday coil too. I was loaned a 13" Ultimate coil and really like how different it is from all other coils I ever used, stock or otherwise, so it's different enough to want one in my line up as well for larger areas to grid faster. I got rid of my 15x12 because of the depth issue *in my soil* and also because it was too much like replicating my 12x10.
If you get a Minelab meter meant to use V-clips, you don't have to mount it in the lousy stock location. Easy mod found in the meters/ID thread sticky. Or Ron's meter doesn't even need the V-clip and can mount anywhere.
Far as warranty/repair work, look at it this way- Some dealers might be selling a new GT a year from now they still have in stock or can get from Minelab's warehouse. They have a 3 year warranty now, so that means at least 4 years down the road they'll have parts. And, I doubt they'd stop fixing them right after that, as that would not be a very good image to customers in the future.