Gears change the personality of our snakes, more than the performance. Changing from a 3:07 to 3:54 will make your car perkier, faster throttle response, faster off the corners, and 6th gear becomes more usable. On the Silver State event I found I could run at 155mph easily in 6th gear, where with 3:07's I needed to be in 5th and wound out.
Super smooth experienced road racers stick with 3:07's as this works well with their style. Nos, supercharged, turboed, or "tuned" engines should stay with stock gears.
Nothing changes the engine, or HP figures, so don't expect tremendous performance gains...just more fun.
That edgy, almost out of control feeling from a gear change is priceless.