Hello Joey and to answer your question the value of 305 Watts is the maximum continuous peak output your power supply can offer when it is put under load.
The only way you can change this value is to replace the diodes, triacs, MOSFETS and TRANSISTERS that are part of the voltage regulator and the various circuits that transforms 115 Volts 60 Hz USA and 230 Volts 50 Hz in other countries, to 3.3 Volts, 5.0 volts and 12.5 Volts for usage for the various circuits found in the computer system.
Most power supplies can accept two inputs, 115 Volts AC or 230 Volts AC, but they both output the same current.
Watts is a working unit for electronic circuits, this wiki can explain in detail what a WATT unit is. http://en.wikipedia.org/wiki/Watt
There are various Zener Diodes on power supply main boards that may have to be changed to a higher value to survive the demand and current flow as well.
To be truthful with you, it could cost you more in parts to upgrade a PSU then it is to buy one already assembled.
By the way, I have been a licensed, certified electronics repair tech for 38 and a half years now, so believe me when I say it's cheaper to buy a new one, unless you build it as a project and know what you are doing, it's not worth trying it.