While I haven't used my 15x12 SEF on the beach much yet, Kered would probably tell you that it's the deepest coil he's used for beach hunting I think, and he's owned a few WOT coils. He also likes it for a few other reasons like small target sensitivity. I'm loving mine, and I have dug some really deep tiny targets with it, but I'm still waiting for it to prove to me that it will get very deep on coins or rings, which I think I've figured out why it hasn't yet (due to me setting sensitivity way too high and blinded out targets). That's going to be true for any coil, though.
What I am hoping NOT to find is that the 15x12 is just taking in too much ground signal and so the effective signal to noise ratio cancels out any depth advantage, maybe even getting less depth than the stock 10" coil. Meaning, the 10" is seeing less ground and so it can handle the sensitivity being set much higher without blinding the machine to targets due to the ground signal. If that's the case then I might step down to the 12x10 coil and that might get deeper than the 10" because it isn't as big as the 15x12. On the other hand, I've read and researched enough about the 15x12 to know that the majority say it is deeper than the 10 or 12x10 on coin sized targets, but again if I don't see that for myself in the field then I'll continue to have my doubts. It might very well be that in my area the moderate to high mineral content eliminates any depth advantage of a larger coil than say the 10" because it's seeing more ground than the GT can handle processing. Some in very bad soils will get more depth with a smaller coil like say an 8" than they would with say a 10", and I'm really hoping that isn't going to be the case for me. Either way, at least at some of my sites the mineral content is very low and so it should still provide extra depth at locations like that. When hunting something worse then I'll step down to the 10" for best depth, and if it's really bad and contains a lot of iron and hot rocks then I'll drop down to the S-5.
