If you just increase engine power, you will get an 'overspeed' warning and the sim will grind to a sudden halt. You need to edit the AIR file using something like AirED (available here). Change the entry in line 316 (Mmo - maximum mach speed) and Vmo (speed in knots) in the Misc section of line 1101 - Primary aerodynamics, to what you want.
THEN you can increase engine power!
Don't forget to back up the file you're going to change first - in case you make an error ...
