That's impressive, i own and tested both the coiltek 17x11 & 18" mono elite coils. Both my coils set to max sensitivity would not go anywhere near that depth unfortunately. I run a stock standard gpx 4500, the 18" mono elite went 38" on a large lump of alloy test piece. So hitting that depth with a smaller coil is brilliant!
I'd like to see a couple of detectors go head to head on that test site, three actually... standard gpx, modded gpx and the Zed