We have our own 991 GT3 demo car and have tried all types of exhaust.
We found that the original manifolds & catalysts are very hard to improve on - unless you wish to remove the catalysts altogether, which would have further issues.
The rear silencer is the easiest to change but the OE design is also pretty hard to beat. It is light and has an impressive internal design that allows two paths, loud & quiet. This component also includes a built-in balance to promote the shriek at 9000 rpm.
QuickSilver used to list a pipe assembly to replace the rear silencer but have now deleted it.
We liaise with Porsche Motorsport in the UK and learned that, in Europe, the various Carrera Cup cars all run the OE rear silencer - it works extremely well, it is light enough (until last year they were using the Titanium version from the RS but this year have reverted to the stainless version as the weight saving was only a couple of pounds), it forms part of the rear crash-zone (regularly used on race cars) and is easy & relatively cheap to replace.
In our testing, we found that the best result came from replacing the OE side mufflers with a tube assembly that incorporates original-type valves - The benefits are:
• Valves continue to operate via the cabin button
• Significant weight saving - over 30 lbs
• Plug & Play - These require no other modifications.
• This emphasises and amplifies the great original sound, and all it's elements.
With this option and the valves open, the exhaust is basically straight-pipe. Sounds fabulous.... Aggressive and sharp with all the crackles and you can turn it down when required.