I was asking this same question here a few weeks back (mainly wanting more depth), I ended up emailing Tesoro and asked them. Here is their full response:
Hi David,
You may get a little more depth with the larger coil, but your detector would most likely need to be tuned to that coil. If your detector is out of tune, you will not get the maximum depth. You can send it to us for a retune, no charge except return shipping....that may help more than a new coil.
You can use any of the "Standard" coils on our website for the Cortes, the first group listed.
If the soil is highly mineralized, a widescan may be the better choice for depth, instead of a concentric. Running the Disc low will help, as well as hunting in all metal, then checking targets in the disc mode. You may already know this.
As for max depth, the Tejon and Vaquero are best. The Tejon is very sensitive in a high mineral environment....most people opt for the Vaquero when the ground is very 'hot'. These use different coils than the Cortes, fyi. You can see field test on these in the MDI on our site.
Let me know if you have any other questions.
Regards,
Casey Stern
Tesoro.com