I just got a GTP1350 and was disappointed on it's depth using the stock 7x10 elliptical coil. With my experience with Fishers, I went ahead and got an 8.5x11DD coil for the GTP1350. Huge difference!
I also got the 10x14 and it adds depth on a larger coin, but I don't think I could swing it. Just too much weight and awkward balance when the detector itself is a little heavy to start with using 8AA batteries.
Great detector, I'm glad I added it to my collection.....
I'm guessing with the GTP1350 recently discontinued, that's why the coils are cheap if you can find them, less that $100 new.
As with the Fisher stock 10" ellipticals, the Garrett 10" ellipticals suck too. This DD coil made a huge difference, even being 7khz, it's only a few inches behind my F70, and about even when the F70 is running in the discriminate mode.
The reason I got the GTP1350 recently? It's got the profiling feature I wished I had after watching a friend use and demonstrate it to me in many hunts, and he made me a deal I couldn't pass up.
I do wished they had a way for it to run all metal mode, kicking out the discrimination circuit. If you scan while pinpointing, I get another 4 inches or so out of it.
In closing I believe there's a reason the few new GTP1350s that are left, are selling for the same price with a crappy elliptical coil as a new AT Pro with the 11DD coil.