904 issue in my new Duster

You should be able to replace and adjust the rear band without removing the valve body.

As far as a shift kit goes... for stock to mild perfomrance applications, a properly adjusted valve body is your best compromise between performance and good driveability. In your application, if I were going to modify the valve body, I would go with a "shift improver" kit over a full shift kit. The improver kit usually only consists of a modified spacer plate (they usually just drill out the front servo/clutch apply orifice) and accumulator spring for a harsher shift.

Furthermore, when manufacturers of shift kits advertize that you get less wear, heat, and sloppiness, they are basing that solely on the fact that shift kits make the clutches engage faster... reducing heat, wear, and sloppiness. What they don't tell you is that it increases the shock on your hard parts... planetaries, splines, roller clutch. A properly adjusted automatic transmission in a stock application should not have any of these concerns.