If you stay under 20 degrees angle of bank for final and climbing turns it means the increase in stall speed is negligible (6.41%) so you don't have to worry about increasing speed. You can bank as steeply as you like - just make sure you have the speed to avoid the stall and power to keep the speed/altitude combination required.
If you don't believe me watch a glider pilot turning final: If they need to tighten up the final turn, they'll roll on as much bank (45+ degrees) as needed increasing speed by the requisite amount instinctively. But then glider pilots do spend a lot of time a couple of knots above the stall in, for power pilots, what is a steep turn.
Bottom line: Keeping to 20 degrees angle of bank in final/climbing turns in the circuit is a good safety guideline - but not a physical/aerodynamic limitation.
OC619