Ditto to what Ken said.
Tesoros get deeper if you tune the ground balance a little negative.
The tuning goes towards positive as you move up in the size of coils.
If you have several coils, you want to send in the detector and have it tuned with the largest coil. This will result in the smaller coils being more negative than the largest.
I have a Cibola tuned for the 12x10. Even with the 5", which is pretty negative, it is stable.
You lose depth if the ground balance is positive....
With that said, unless the seller sent it back and had it retuned with the 11x8, it's tuned for the stock 7" that came with it, making the 11x8 not operating at its peak performance.
How much more depth could you get with the 11x8 tuned instead of mistuned? I wouldn't lose sleep over it though I don't remember what the difference is. I can't find my notes when I retuned my Cibola. Someone could probably take a Vaquero or ElDorado or similar and give you an idea by tuning it for a smaller coil and then comparing the mistuned 11x8 depth with the depth of the 11x8 then tuned.
Guess it depends on what you want to hunt. The 11x8 coil costs a lot more than the smaller coils....
/end rambling....
