I hope you don't mind using a sling, any 15" coil will make it a heavy swing. I have a 15" butterfly coil and can swing it for about an hour without a sling. When I first got my F75LTD I decked it out with a larger coil and a probe. Man it looked badass, but I find I like a the convenience of a pin pointer over the probe and I also rather have better target separation rather then the coverage and depth of the larger coil. I normally have the nel sharpshooter coil mounted unless I am in the fields.
The lions share of my bucket list items and other keepers were between 4 to 10.5 inched deep.
I have not heard anything good about the Fisher 15" coil, you'd be better off reviewing aftermarket coils IMHO
GL & HH