I always use the coil that comes with the detector. I figure the manufacturer wants to get the best public image he can get so he designed the best coil for the detector. The only advantage of a smaller coil is in extremely Junky areas, which usually do not contain gold.
My XLT and Surfmaster PI both have the original 9.5" concentric coils and they have found just about everything a detector can find. My 2 Box detectors 2 coils are actually the metal trim around the cases. Yea, that caught my funny bone too when I noticed it. Hay that 2 Box, a Hays, will go down 6', but only for something larger than a soft ball. Your detector, if it can tolerate the mineralization of the ground in your area, will find anything most other detectors will find, but it won't go as deep as the real good ones. Jest my thoughts, Frank...-