I had an NEL coil for my AT-Pro a while back. Sold it to Mattmit. If I remember right it was $265 shipped to me. I really didnt notice much improvement vs the 8.5x11 coil. Maybe I just never got it over anything really deep but I sure did cover ground quicker.
Somewhere recently, I cant remember where, I heard/saw a video talking about NEL coils and the Max. One of the guys NEL coils worked perfectly on the Pro and the Gold but didnt work at all on the Max. ??? One of his other NEL coils worked on all three. So guess you're rolling the dice wether your specific NEL Coil will or will not work on the Max.