Technically it takes the same amount of "stored energy" (elastic potential energy) to hold the front end off the ground a given amount regardless of wheel rate (spring constant "k" in Hooke's Law). It's a function of front end weight.
In a launch situation a softer set of bars may have more (angular) preload remaining when the UCA hits the rebound stops, where a set of stiffer bars may be completely relaxed before the UCA hits the stop. So the softer bars may not actually release all of there potential energy.
Smaller bars are physically lighter which helps. A set of 90/10 drag racing front shocks will help let the front rise on launch.
