There is nor reason why they would change the old GT1 engine of the Turbo. The Carrera M97 engine has nothing to do with the turbo/gt3 one.
And it is said that they had in Weissach a turbo with more than 520 HP. Thus, there is no reason for them to change anything. This additional 30-45 HP more from the base 480 will be sufficient.

Anyway, the Turbo S will probably be not available for quite some time.
Everyone will go for the base model. Then they will release the PK, so everyone who is out for the newest model has a reason to switch again in 2007. The S version will just be available after some years to keep interest alive.