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
SOUND ARCHITECT
Electronic/Pneumatic Valves - Retains OEM (in-car) functionality
Our active valve sport exhaust systems contain two separate gas paths, and an electronic or pneumatic flap-valve that decides which path is taken. Both have two distinct levels of sound and are slightly louder than stock. With the valves closed, the system takes on a more civilised note and when they're open, you're given a sporty note throughout the rev range.
These valves are integrated into the car's existing controls and function depending on the ECU mapping. This can then be further manipulated either with the dedicated exhaust button or by switching the driving modes.