I'm no expert in transmissions, but I am aware of pitfalls of increasing line pressure to improve shift feel in some transmissions. I believe this is done is some shift kits. I also believe this type of shift kit caused Ford'* AODE to fail when people added these kits to their tranny'*. The higher pressure would blow seals and thus the transmission.
The other method I know of was to increase the bore size for the shift solenoids so that the same pressure would fill a solenoid faster. They would also recommend different springs. Because the line pressure is not increased, seals are not blown.
I'm not actually sure what shims do in the 4t60e transmission. I'm not sure if they increase how fast solenoids fill, or if they somehow increase line pressure, or if they do something else. Perhaps some one can tell me how shims do improve shift feel.
I also would never play around with this unless I had instructions from a very knowedgeable person.
If I am in error with any of my information, please tell me. If my information is likely to cause some one to muck around with their transmission and blow it up, my post should be removed because I don't want anyone to damage thier tranny'* on account of any information here.