I'm a nugget hunter out here in Arizona, and have also been curious about getting an upgrade coil for my MXT. Does anyone have one of these Ultimate 13" coils or Kellyco SEF Excelerator butterfly coils, which I feel is what White's may have tried to copy with their Ultimate coils? The Eclipse coil that came with my MXT as the stock coil doesn't cut it for use in highly mineralized ground areas, so am wanting if not NEEDING to upgrade to a much better hotter coil, for MUCH better overall performance in highly mineralized areas. I've sent Kellyco numerous emails asking them about their recommendations, but to no avail. Normally if a company won't respond back, their products aren't worth a plug nickel. There is no doubt about Kellycos customer service to be greatly lacking!!! My main concern is not wanting to waste money on a coil that isn't as good OR BETTER than the stock Whites coils that I already have. If the Ultimate coil won't do, then how about the SEF butterfly coils, and which one is best for overall hunting/performance? Which SIZE butterfly coil - 12x10, 15x12, 13" Ultimate? Sharing your recommendations and experiences with these coils is MUCH appreciated!!!