9mm+P is a SAAMI standard, and is equivalent to 9mm NATO loads
Calling it overpressure is less than correct, even more so if you're talking about European firearms that are designed around NATO spec ammo.
Calling it overpressure is calling it what it is. It is over the standard pressure for that cartridge. A 9mm Luger+P is an Over pressur Luger round, it is not the exact same thing as a 9mm NATO round.
Wikipedia
http://en.wikipedia.org/wiki/Overpressure_ammunition#.2BP_ammunition_use"Official +P pressures are established by the SAAMI for certain cartridges; in general the +P pressure is approximately 10% higher than the standard pressure (see chart below). SAAMI does not have a +P+ pressure standard, but this indicates a pressure higher than the +P loading.
Cartridge Standard pressure +P pressure Notes
9 mm Luger 35,000 38,500 10% increase
.38 Special 17,000 18,500 9% increase
.45 ACP 21,000 23,000 9.5% increase
.38 Auto 26,500 36,500 38% increase to make .38 Super
.45 Colt 14,000 27,500 96.4% increase, For use in certain modern revolvers and lever rifles
.257 Roberts 54,000 58,000 7.5% increase"
Article @ Gunnuts
http://www.gunnuts.net/2009/04/03/9mm-nato-vs-9mm-luger/