The 10 Speed is solely for fuel economy. Even a 6 speed is solely for fuel economy. 5 speed transmissions many times have the same gearing but with one less overdrive gear.
In fact Motor Trend stated that a 5.3L Silverado is slower with the 8 speed transmission than a 6 speed transmission. The extra gear changing, that's not overdrive gears, takes extra time obviously.
The one maker who really needed a new transmission was Dodge, but even with it's 8 speed transmission, the Ram 5.7L is still pretty slow next to any of the F150s and previous gen F150 3.5L Eco, 5.0L, 6.2L trucks, and current GMs 5.3L and 6.2L trucks. The old trans had poor gearing for performance. The new trans fixed that, but the extra gear changing didn't make it much faster.
