Here's what I found on Motive's website:
The Motive Gear Stock Performance is an excellent high quality light duty O.E. quality replacement ring and pinion. It has 8620 steel, CNC throughout and computer controlled heat treatment. Many installers have reported that this ring and pinion series seems beefier and quieter than OE gears. An excellent choice for both on and off road applications. These gears are found in everyday automobiles, trucks and vans. These gears are tough enough for UROC racers like Craig Stumph and Bart Jacobson.
Motive Gear Performance High Performance Series has more features built into it on top of the 8620 steel, CNC throughout and computer controlled heat treatment as the Stock Performance series. They are also double lapped, have index tooth spacing and central contact patterning. This High Performance Series comes in a huge range of ratios. These gears are used in stock car racing, drag racing, the toughest off road racing, sports cars, muscle cars, trucks and everyday automobiles when the driver demands the highest quality and performance.
Does anybody care to elaborate on this? What are the advantages of the double lapping, index tooth spacing and central contact patterning?