The SEF coils mount dead center. Much better balance than say the 10" coil that has the shaft mount more towards the back. It makes them less nose heavy, and balance can be just as important as weight on a machine.
My theory is that the 12x10 will see less ground than a 12" or probably even a 10" coil. The left/right DD detection field on this coil seems very tight. The 15x12 was an improvement over the stock 10" coil in that respect, but you can REALLY feel this razor sharp edge on the 12x10. It makes my GT react faster, separate easier, and almost feels like somebody turned up the clock speed of the detector. If I didn't know otherwise I would have bet money on that had you handed me a mystery Sovereign with this coil on it.
Anyway, since the left/right field this thing generates seems much tighter with less "slop" off to the sides my theory is it's stewing and soaking in less ground matrix. Results are higher sensitivity and probably better depth with less target degradation. Although these FBS and BBS machines are light years head of the competition in terms of ignoring ground minerals, they are still subject to bad effects from them when the minerals get real bad. Thus my unproven as of yet theory (just got my 12x10) is that you'll get deeper and see targets that aren't washed in as much ground matrix, meaning deeper/cleaner target IDs than say a 10" or a 12" conventional DD round coil. That's where you'd probably see the 12x10 take the lead in moderate to high mineral sites, where as in low mineral locations they probably are very close to each other.
I'd still give the 12x10 more points for better separation, pinpointing, and stability. The audio also seems much crisper and reactive. In fact, on my GT it sounds like the audio pitch has been turned up a fair bit on targets. Never would have guessed changing coils could do that to the audio. I suspect the tighter left/right field makes some how for higher pitched responses over targets. Now my silver hits sound as high as my old QXT Pro. Good, I've trained my ear more for that. Not that the stock coil or 15x12 doesn't give the kinds of tones I like, it's just that the 12x10 makes them a little higher where I'm used to it from prior machines.
I'll say this...Although the 15x12 was an improvement in all respects over the already great stock 10" Tornado (A much better coil than the 10" coils I had on my Explorers), it did not go as deep on land due to my high minerals. Sure, it would often run at higher sensitivity settings and be stable, but in my soil max stable sensitivity doesn't equal best depth. Just like on my Explorers, I have to bury a dime and calibrate that way. Too high can degrade or even null out targets, even if these machines seem to run fine. Doesn't matter which coil you are using, either. The 15x12 did go much deeper than the stock coil in sand though. Can't figure that out because my beaches are highly mineralized too. Anyway, that's why I traded the 15x12 for the 12x10. My theory is that this coil can push a little deeper than stock, where as a conventional 12" coil or bigger will be seeing too much ground matrix and get even less depth than the stock 10" coil. In fact, some people in high mineral areas get more depth from an 8" coil because of this. Less a problem on BBS and FBS machines but still an issue once you pass certain limits.
Pinpointing on the 15x12 or 12x10 is better than the stock 10" coil. Probably better than any machine I've ever owned. Use the tip or the tail and wiggle away from and towards the target. As soon as you hear it it will be at the base of the "V" the coil forms at the top and bottom edges. Repeat from 90 degrees and it's dead on. So good I hardly use PP mode and just use discriminate to PP targets. Also deadly for picking a coin out of trash where as regular PP mode might sound off to trash or high ground minerals. Many times PP mode can't hear a deep target for me, where as it's easy to hit and PP in discriminate.
I also love using the tip of these SEF coils for investigating multiple targets. Wiggle the tip at the targets to try to splice them apart. These SEF coils seem just as deep or deeper than conventional DD designs from tip to tail, but they also seem to have a hotter concentric center to them about the size of a soup can or so. That's why I don't like using the center to PP, but I don't even use the center on my stock coil. I prefer using the tail on that. Tips for the SEFs on land, but I also use the tails of them in the water. Stick your big toe in the "V" and that's right where you need to scoop. Although these things seem real hot towards the center, they'll still separate targets very well swept over dead center. It just goes from good to amazing when you use the tip or tail of the coil to split targets apart you never would have thought you could do otherwise.
Although the 15x12 would often allow much higher sensitivity settings than the stock coil, when I did my buried dime test more often than not sensitivity would end up much further down the scale, roughly where the buried dime test would have me set the stock 10" coil...About 2PM at most sites, perhaps higher at others. That's even though max stable sensitivity could be say 10PM (it goes counter clockwise on the Sovereign to highest) or even full max manual without falsing. Now, with the 12x10 I'm finding that this coil will also go to max sensitivity without getting unstable, probably even at more sites than the 15x12. But the real interesting thing is that with the buried dime test my sensitivity usually now ends up being much higher than it would be for the other coils. 9 or 10AM I'm finding at most sites so far, a much higher sensitivity for "best/easiest/most stable" ID on the buried dime.
I stick the dime 6 to 9" deep and then check it by wiggling or short sweeping over it. Often too high will cause the ID to wander or be erratic, or even null the target. Too low can also null it or make you work harder to wiggle the target to proper ID. Once you get the dime at fringe depth (for that site) you'll find a very specific setting on the dial will get proper ID with little effort, hold it there easier, and won't jump randomly on you.