I've been using the SunRay Invader 12" for many years on my CZ3D. It has greater depth capability than any of the stock Fisher coils. I average about two more inches in depth over the 10.5". Many years ago I had gone back to parks and worked some of the same grids I had previously worked and wrote off with the 10.5" Fisher coil. While going back and hunting these same areas but now with the SunRay, I dug mostly targets that where deeper than the first time. Most of the nickels,dimes and quarters found the second time around where at 10-13". There are are less coins at these depths than most people realize. Most of your coins will be found in 8" or less while working in sandy soil. Loam and clay soils will obviously be different.
There are a few trade offs. The target i.d. will be slightly less accurate and working in trashy areas can be a pain unless you slow down. Its heavy and you'll want to either chest or hip mount your control box. The SunRay coil as previously stated will go deeper ( depending on your soil and conditions), its wider and will quickly cover more ground. It pin points better than the stock coils. When working with iron masked targets it is about the same as the 10.5 Fisher, you need to circle the target and learn how to listen/decipher the tones. I don't mind digging deeper targets but it can be an arm breaker when you dig four or five deep ones, one after another, I use a small sharpened shovel as I have negative motivation for using a hand digger while hunting with this coil. At the beach, I've dug deep clad quarters in damp sand on a pretty consistent basis at around 15 -18 "
Don't let everyone confuse you with air tests. The air tests will yield a positive greater depth over the 10.5 but your soil type and soil conditions are reality and will either work with you or against you no matter what you are swinging. The real question as to if a larger coil is worth the money and if you will benefit from it is very subjective. I find it most useful in heavily hunted areas where I can find targets that were missed by other hunters.
Good Hunting to you and Long Live the Republic.