456 WHp sounds kinda high for 7 psi. My guess is the manifold pressure was a little higher.
Shift kit and trans cooler both keep trans fluid temps down. The fluid is actually heated up while the transmission is in between gears. A shift kit shortens the time to shift from one gear to the next minimizing fluid heating.
Freeway cruising in over drive is not stressful on the transmission, but having the AT kick down on it's own under boost is bad. When driving a boosted AT ... always manually downshift before flooring it (and don't forget to hang on
).