I got a cousin, a Cors Cannon coil for my F70, very similar to the Nel Thunder even though I already had a Fisher 11" DD.
Heavier than the big Fisher a little, knocked my very ergonomic detector out of whack a bit but nothing like the big 15" Fisher coil that is way heavier, way expensive and very few buy.
They had to get rid of some of those by giving them away for free in a package deal with a new F75 a few years ago.
I assume a whole lot of them are taking up valuable space in a warehouse somewhere.
Even some that got them free complained they were unwieldy and bent their lower rod when mounted.
My Cannon coil is doable, don't really use it all that much because where I hunt depth isn't as important as unmasking abilities so snipers are usually my go-to coil of choice.
Think about all that, if the Thunder is that big and I can definitely notice it on the almost perfectly balanced F70 what would it do to an AT Pro which isn't quite as ergonomic or what would the even bigger coils feel like when you are swinging it for hours.
Those way bigger coils they make, I am in no mood to swing one of those around on a hunt but like I said, I am not looking for shocking depth where I hunt.
If you are lugging those huge coils around could be a wash if your body can handle it and it actually could help you find the deeper targets you are looking for.
In farm fields I would think you would be looking for great maneuverability but in corn stalks and other vegetation hovering over areas with bigger coils might make up for the space you would lose from ground level searching.
Fishers have a boost feature to deal with that stuff using all coils, nothing like that on the AT Pro.
I hunt parks mostly, or course those very huge coils would be way overkill in those kind of sites.
The actual reason why I got the Cors coil is because my F70 is majorly powerful and real sparky so chatter is just expected on higher power settings...which I mostly use.
The Nel and Cors coils are much better shielded than any factory coil put out by FTP so much quieter and more stable...noticeably.
I was most impressed with that as it proved true on mine using both that Cannon and a Nel Sharpshooter.
Still boggles my mind that if a foreign company can do this surely American factories could too but in the case of Fisher...they don't.
As far as targets bigger coils can get deeper than smaller coils, maybe, but a lot of that depends on site conditions like soil types, moisture levels, EMI interference and more.
Measure any gains in inches, sometimes just one or two if it works well so don't expect miracles but sometimes just one of two inches might be the difference between noticing great targets at depth or not.
Also if you are looking for tiny things the bigger the coils the less ability they have to do that and separation could get a little less sharp so if you have major masking issues a way big coil might not work as well as you would like.
A button next to decent sized iron get lost compared to smaller coils in the right situation.
The good news is my Sharpshooter scores high in depth, separation and noticing really tiny targets but it should being a sniper coil and all.
The better news is the bigger Cannon coil matches it in all those areas, I was surprised on how small the targets it could notice and the separation was extremely good for such a big coil.
Depth was probably a bit better than my 11" Fisher DD too according to some readings I got using pinpoint especially when I tried it in good, dark soil.
Didn't check a whole ton of them out, however, because digging 12" holes in public parks just isn't a good idea.
Weigh your options, consider the weight, maneuverability and everything else I mentioned and use common sense if you are going to spend the money on one of these things because they ain't exactly cheap.
Hopefully you will choose well and it will do what you need as you are expecting it will.
Good luck!
Oh yea, PS....
Search through some of the Hoover Boys vids on youtube.
There are a couple where they are using some pretty huge coils on their AT Pros in farm fields so if you can find them that might give you an idea on how well they work.