GM didn't intentionally design the bracket to flex for improved fuel economy. It's just a cheap design that allows flex and hampers accurate throttle control. The true source of the unresponsive throttle is the drive-by-wire system in conjunction with torque management:
"The 5.3L, and 6.2L suffers from poor throttle response down low. In fact the factory programming has limits on the max throttle blade opening that PCM allows and is based on RPM range engine is operating in. By correcting this issue and a few others; improvements will be much better throttle response, step on go pedal, vehicle will respond quickerthis DBW system is a torque driven system, it works by taking a certain amount of torque via engine and wheel and applies it to what the driver foot say it needs; a better way to explain it is its a torque limiter. It only allows so much torque at a given pedal position. Stock tables are conservative and numb at best and thats why one would feel some times a lag or a goofy pedal with DBW systems. For a 5.3L, or 6.2L V8 this is one of the most critical areas that must be addressed and modified to generate more torque /hp and throttle response."http://5startuning.com/got-a-gmgmc/
The pedal flex eliminates some of the vagueness of the throttle but only a tuner can make your truck truly reponsive and "torquey" off the line.