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.
*This system does not fit the 991.2 with OPF filters