Knight, I'm actually on an aerobatics course right now where we do 75+ degrees bank turns with full power. Just keep pulling and increasing bank, maintaining altitude, until the aircraft stalls... :-) (Do not try this at home though.)
During PPL training, the 20 degrees bank was the limit when doing the turn to (short) final, with (almost) full flaps, low airspeed and low power. At least, you had to plan your final turn so that it could be achieved with 20 degrees bank, whereas the downwind to base turn, with more speed, could be achieved with 30 degrees bank.
Obviously it's all got to do with g loading. And with nervous instructors, too...
Even my aeros instructor (who happens to be a display pilot too) gets rightfully nervous if you start manoeuvring aggressively, on final, with full flaps, low airspeed and low power settings.
Still, if you planned your turn to final wrongly and 20 degrees doesn't cut it, like you say, it's better to put a bit of power on and roll on 45 degrees of bank, than to try and force the aircraft around with the rudder while maintaining 20 degrees.