This is a complicated question :
The RIGHT answer (since I know everything in the world, and if you doubt that, just ask me) is:
On ALL metal detectors : The all-metal mode goes deeper than the disc. mode. Thus yes, you will go deeper (and thus find those deeper coins) IF you go into all metal. However, the devil is in the details: Because the moment you do THAT, is the moment you lose the ability to knock our iron . Doh ! So heaven-help you if you try this trick, to gain another inch, in an iron-ridden environment. You'll go wacko.
The only reason some people might THINK that their disc. mode goes just as deep as their all-metal mode, is that with a lot modern detectors the 2 modes are running simultaneously . Leading you to THINK that you are getting the same depth. But the truth is, that those fringe targets are not registering their TID. Yes they might register as an iron grunt or whisper, but are not coming up into the disc. range.