I bought a 10" concentric from Bart and it was cheap.
Didn't use it a ton, I preferred and mostly used the sniper because I hunt a lot of trashy sites.
I can tell you it was deeper than the 8" coil by a couple of inches and I got clearer more solid signals on most targets at 5" and deeper.
This was in excellent black low mineralized soil in Kansas.
Only had about a dozen hunts with that coil, if that, but one of those hunts produced a gold pocket watch so it did its job and then some as far as I am concerned.
Couldn't see the logic in buying a Fisher DD, Nel or any other expensive coil for a lower end unit although I have read about many that did and got really decent and improved depth so they seemed happy.
A few got the Fisher DD coil and didn't see much of a difference from the 8" at all so they weren't happy.
A lot depends on your soil on how much benefit you can get from upgrades like these.
Eventually I upgraded to an F70 with way more depth than the F2 could hit with any coil and put away the F2.
Loved my F2, I wrote glowing reports about it and my finds for 3 years, but if you want a real upgrade save up for an F5 or an F70...even an F75 since you already know a lot of the Fisher language.
Way more power, tons of settings that will conquer just about any soil and conditions.
You can get them hardly used at discount prices if you look around but even new they will all pay for themselves.
