What other coil do you have for your machine? With the same conditions and the same test coin, what does your other coil do? I wouldn’t consider that size a “large coil”, and didn’t even know that size was even made for FBS. I’ve heard of the 12x10, not the 12x8 though...a “large coil” would be +2” over the 11” stock coil, such as a 13” ultimate or even the 12.5” Sunray x-12 or the 15” WOT from Coiltek....or larger. The field generated by a 12x8” coil will be about the depth of a 10” DD round coil, if that. I always hear, “Wow that coil punches deep”....but there are two things, and ONLY two things which dictate operational depth. Those are the configuration/size of the coil itself and the sensitivity setting, when talking about FBS. And...the WIDTH of the coil is generally the DEPTH at which a dime will be seen with decent reliability in moderate soil.
IF you are looking to go “deep coin hunting” get a Sunray X-12, an Ultimate 13, Detech 12x10, Coiltek 15”, etc.