How to detect very hard impacts with piezo?
Yes, the output of a piezo-electric sensor should keep increasing as the target is whacked harder.  There should be a substantial difference between tapping the target with your fingers and hitting it with a hammer or a bullet.  One problems with piezo sensors is to keep them from frying the input stage if accidentally whacked hard.

Of course, the obvious thing to do is simply try it.  Set up something that triggers if a signal gets over a couple of volts or so.  Then feed the piezo signal thru a pot so that you can adjust the attenuation, then into the comparator.  After some experimentation, you should be able to find a setting that detects hard whacks but not casual bumps.