I live in Japan, own two CTX3030's, and have to deal with the most mineralized soil on the planet! One of my detectors is two years old and the other is 3 months old. My original stock coil that is 2 years old would only penetrate the ground 4 inches with a sensitivity setting of 19-21 before it got crazy noisy and unstable; Minelab replaced it under warranty. The new CTX stock coil they sent me still only works to about 6" before the same thing happens, but now my old and new CTXs detect to the same depth and are stable at about 23 sensitivity. I also have a 17" coil that I received for FREE with my new detector. That monster is even less stable in this soil - essentially unusable. Why am I telling you all of this...I personally think you have a a bad stock coil. I recommend you call Minelab, explain that the stock coil is struggling in mineralized ground, but your 17" works just fine. Ask them to send you a new one under warranty and send yours back. There are a number of posts out there regarding bad or weak stock coils. I am confident Minelab will work with you - they are very responsive.
Bottom line - if your soil is mineralized to the point where the stock CTX coil is struggling, then your 17" should NOT be deeper or more stable - it should be worse than the stock coil in just about every application. Essentially, big coils are traditionally more unstable in mineralized soil than small coils because they force the computer to process MORE unstable data. My 6" is more stable in my mineralized ground than the stock coil, waaaaay more stable than the 17" and routinely capable of seeing coins at 8+ inches. The trade-off.....the 6" is not going to help you cover a lot of ground in a limited amount of time. It is a surgical instrument! HH - Shayne