They definitely break in, and it's easier to notice the difference in the drivers due to their sensitivity to PLH changes (which is why they had trouble with the drivers initially). Best example is my first edition Shock...When I first got it there was some stability there. It had some turn, but still seemed more overstable than most my Volts. After bagging it for a couple of weeks and getting some hard hits it now has Amp HSS. Also, my stormtrooper Amp started out as stable as a Volt, but it has worn in to understable faster than Volts I have that started the same stability.
Drivers want to behave according to their shape(either mold shape or with extreme high or low PLH), but for some reasons sometimes new discs have added overstability that does not last as long as if it were truly an overstable disc. I prefer fairly neutral distance drivers, so that leaves me needing to disc up in the wind. I don't usually like adding a mold just to fly straight in the wind, so usually I've found an unusually overstable freak disc of the same mold as my primary D driver. No matter what the manufacturer or mold, I always have to protect my "overstable" stable distance driver because it only takes a good hit or two to have the disc acting more like it's non-freak brethren. I used to attribute this to flash, but it has happened multiple times with discs I deflashed before the first throw. This is of course my own experiential conjecture.