That's right, they raise the pressure in the lines for harder shifts and they change the point in the RPM-band and speed where the car shifts, whereas the spacers included in a shift kit manually do it. Either way, the 20's aren't going to be easy on the trans, with out without a shift kit and with or without a tune with hardened shifts and changed shift points.

The best thing to do with 20's to keep your trans running for a while is drive slow, don't beat on it where there would be tons of pressure while the car if shifting. Do that and you should be okay for some time, do expect it to go out sooner than it would with stock wheels though, it's going to naturally be more strain on the trans.