This is one of my past dogs at working weight. He had unlimited energy.
This is the same dog for show, weighing 15 lbs more. The extra weight clearly affected his performance on the field while he was heavier as he ran out of energy way faster than when he was at working weight.
My experience in showing and working is extra weight puts stress on the dog's joints, but in show, they do not like to see the dog tucked up so they need the extra weight to lose the tuck. When this dog was at working weight, he was SG rated. I put extra weight on him and he was V2 working males at the 2004 USRC National Sieger Show in New York.
I think diet is critical for any top performance - show or work and there is also a difference in weight on show and work as different requirements are being asked from the dog.