shims only support it where they are installed, dry packing the base will give support under the complete base. If it had been installed properly, the drain pipe in the floor would already be 2″ so a transition would not be necessary. Anything that goes under the shower, unless it is on an upper floor or has access below it, has to be installed prior to setting the base. The plumber knows the sequence to install it, so just give hime the base and turn him loose while you go out for a cold beer and a ball game.