I am compelled to comment on this, since I’ve made sort of a opinion shift of late on this topic. For most players, I agree that better swing technique, more consistent impact factors, and improved fitness are the most important keys to improved length in todays day an age. However, with your better amateurs and professionals, I also believe that having the correct equipment that suits your body and swing can make a huge difference.
For guys that have a reliable and repeatable swing, having the correct equipment and (most importantly) shaft can make a huge difference. Finding the right technology to maximize your launch angle and spin rate of the golf ball will not only inherently improve overall distance, but will also lead to a more predictable ball flight. Spin can be a killer of both distance and direction for the majority of miss-hit golf balls, and finding the equipment, along with the right technique, to find the “sweet spot” for spin rate will go a long way in improving both. I believe a good fitting for your better players can make a significant improvement.