Nick - I find my GT4 is soooo much louder than the 997 GT3, don't know how it compares to a 991 though.
You're right, a GT4 feels like a hooligans car. It feels like an old fashioned hot-rod to me - such a naughty car I'm surprised they built them, almost un-Porsche-like in the fun way it drives.
At the end of the day, I do most of my driving on the roads, incl Euro hoons, and the GT4 is more fun on the road, and more accessable on the road than my GT3. I do blat around tracks too, but I don't time myself, so it's about fun and smiles rather than outright lap-times....here, the GT4 delivers.
Walter recons drive a GT4 in the mountains, and the GT3 on track - I couldn't agree more.
Hopefully you will have the same experience as me....it's a riot of a car!