Hey McDave --
Just FWIW -- I ran my new 6x8 S.E.F. coil through my test garden yesterday. Results? It is roughly 1 1/2 to 2" less in depth, in my soil, than my Pro coil. A 10" quarter in my garden, with my Pro coil, is a non-diggable target, but it will give an occasional, peep/chirp on myabe 5-10% of sweeps. This is the same response I get from an 8" quarter with the S.E.F. coil. Likewise, a 6" quarter in my test garden hits on the 6x8 (very diggable) about the same as an 8" quarter with the Pro coil (very diggable).
A 6" memorial penny is a pretty good, diggable target with the 6x8 in my garden, though it's getting near the limits...FE numbers are high on that 6" penny, but the CO numbers are pretty good. FWIW, I took it out hunting after testing in the garden, and dug a 6 1/4" memorial that hit exactly the same way "in the field" as the 6" penny in my test garden. A pretty good, diggable signal, but jumpy/generally-too-high FE numbers.
In addition, this coil pinpoints well, especially in trash -- as it allows you to "lock on" to more closely co-located targets and pinpoint them (as you would expect).
Looks like it will be a good coil. Whatever depth you get on your Pro coil in your soil, subtract about 1 1/2" to 2" and that's what you should get with the 6x8 (based on my testing). Please keep in mind that my soil is apparently challenging for detectors -- while most folks consider it "mild," it seems to be tough dirt to detect in, ESPECIALLY when dry (which it usually is here). So, take my depth values themselves with a grain of salt, as you may get deeper targets in your ground. But bottom line is -- for me, 6" coins are very diggable with this coil; you MIGHT be able to sneak an 8" quarter or half. For me, with the Pro coil, it's the same thing, but adding about 1 1/2". In other words, a 7" dime/8" quarter is diggable; you MIGHT be able to sneak an 8" dime/10" quarter, but that's pushing it to the max. So, 1 1/2" to 2" less depth on the 6x8 versus the Pro coil, but good target separation/pinpointing, and comparably-good VDI numbers.
Hope this helps.
Steve