Capsule versus metallic sprinklers
Your reasoning is completely off-base.  I've been in a building during a sprinkler activation (due to mechanical violence to a capsule-type head) and only that head went off.  (Deluge systems used to protect extreme hazards use open heads that have no capsule or metal strip at all.)

The "automatic overpressure pump" you mention is likely actually a fire pump used in tall buildings to make sure that the standpipes and sprinkler systems have enough pressure, as the city water pressure isn't high enough to get large volumes of water to the tops of skyscrapers.  Even then though, the pressure generated by these pumps is limited by reducing valves in order to protect not only sprinkler heads, but firefighters who are using lines off the standpipe system -- this was demonstrated dramatically when some of these valves were found failed (see document p.10, PDF p.20) by firefighters battling the legendary First Interstate Bank high-rise fire in LA.  

In short -- no normal building situtation will expose a sprinkler head to pressures above the typical 175 psi rating.  The city doesn't supply water at that high of a pressure for obvious reasons (faucets don't like 175 psi any more than sprinkler heads do!), and fire pump systems in high-rises have reducing valves going out from the main riser to each floor's sprinkler trunk to keep the fire pump from damaging the system inadvertently.