A Garrett Groundhog fitted with that 10" DTex coil was a formidable combo back in the early/mid-80's. Ken Roberts of Finders put my then wife and I on to the set-up - the DTex coil wasn't as solid or heavy as the more expensive Garrett equivalent, but what it lacked in those departments it made up in sensitivity that helped us find heaps of tiny nuggets.
If that coil and detector are still functional, I wouldn't bother thinking about other coils - what you've got there is as good as it gets!