B&M also sells a "shift improver" for the 4L80E. It too just increases line pressure by fooling the computer.
Before you go buy one though...
All of the shift "improvers" that are not installed in the valve body (reprogramming kit) simply "fool" the computer into thinking the line pressure is lower than it is. Thus, more pressure is "allowed" by the system giving you harder, faster shifts. It works well.
However, these $40-$50 devices are nothing more than two resistors. You can accomplish the same thing with a potentiometer (adjustable resistor) connected to the same ECM wires used by the shift "improvers". Then you have an infinite array of adjustment.
BUT BE CAREFUL! Too much line pressure and you will blow the case apart! Trans-Go has a kit that fixes a known overpressure problem with the 4L80E. By design, the 4L80E is supposed to "dump" extreme pressure and thus save the case. However, there is a chance that the factory valve will stick, causing the case to blow.
If you don't have the "fix" for the overpressure valve installed...be cautious.
Transgo kit 4L80E-HD2 notes:
Ends direct clutch burnup. Holds BIG horsepower. Holds 1st, 2nd, and 3rd to any speed. Backshifts to 3rd, 2nd and 1st at any rpm. Prevents broken case and drum due to high pressure.