Like the title says, just a rumor and I pass it on.
The source isn't too reliable but it provided one or two pretty accurate infos in the past too, so I just pass it on and you guys decide for yourself if you believe it or not.

Turbo gets 480 HP, engine based on the GT2 MkII engine, just a few things modified to improve emissions, noise regulations and to adapt the engine to ESP/4WD, etc.
3.6 l engine with two turbo chargers, two intercoolers, etc.

Turbo S gets 520 HP, engine based on a slightly new generation of turbo engines which of course is also based on the "old" 996TT/993/GT1 engine design. Displacement is very likely in the 3.8 l to 4.0 l range. Turbo chargers seem to have a "cooling system" (water, oil, whatever).

Turbo S gets another new(?) PCCB generation with 370 or 380 mm brake discs front, 355s rear.
There is also the possibility of various engine programs, for example one to reduce power(boost) to save fuel or another one which delivers some sort of overboost for a few seconds, increasing power to even more than 520 HP.

Again: RUMORS!!! There were more details available but due to reduced credibility and/or the likelihood that if they're true, they may unveil our source, I left them away. Sorry.

And now let the speculation begin.

BTW: the 997 Turbo will NOT be at the Frankfurt Autoshow.
Introduction very likely in Geneva or Detroit(Geneva is more likely, Detroit will host the GT3).