The Gemballa exhaust doesn't sound like the Porsche sport exhaust, they never sounded like the classic 911 exhausts but always delivered a throatier and louder sound only.
It would be an alternative to the PSE but personally, I'd go for the original thing.
I also don't trust the 30 HP claim (I think it is only 25 HP over here in Germany): the 997 Carrera S is a normally aspirated car, to achieve 25-30 HP more, you need a little bit more than a new exhaust, highflow cats, etc. A motronic software update might get you than in conjunction with the exhaust mods. I'm always a little bit sceptical regarding Tuner claims: maybe (just a speculation!) they took a 997 Carrera S, installed their exhaust kit ("powerkit") and they measured 380 or 385 HP on the dyno. But maybe the stock car already had 375 or 380 HP...
Just a good advise from past experience: stay with the PSE.
When it comes out and you don't like the sound, you still have time to go for the Gemballa system.