The stock retainer for the rear servo is very thin, and often gets bent. The kit from mrrandy comes with a much thicker, stronger, retainer.
IMO, I would use the new spring; especially if that is the one from mrrandy. It should be, and apprears from your picture, to be stiffer than the stock one. That should help the band release during the 1-2 shift (low band apply valve bodies).
As far as the trans shop taking the shims out of the servo, that stock configuration will cushion the servo/band apply. And since the stock valve body makes real high pressure going into reverse, and the rear servo gets engaged in reverse, they might be trying to ease that application.
This is why it is important to go all the way to idle when putting a torqueflite in reverse. However, in a race application, shimming it solid will improve the servo release during the 1-2 shift.
