There is a SAE paper on ATF+4 performance versus the three other common trans fluids

http://dastern.torque.net/techdocs/nonlamp/982674.pdf

It is worth noting that ATF+4 will allow clutches to slip earlier.

See Table 10 and Figure 9

This is probably only important when friction materials and clearances are worn.

Other figures in the paper show that ATF+ has an improved life, and has special torque converter "shutter" characteristics.