Tbh, fitting standard GS boxes to a standard 4200 system will be at best disappointing. You will get no real increase in sound. What you will get is an increase in bass at tick over, which gives the impression on more noise but it really isn’t. To get a noticeable increase in sound levels, you need to change 2 or 3 of the exhaust elements, these are:
- Cats - change the standard 2 x 400 and 2 x 600 cell cats for 2 x 200 cells.
- Centre resonator - change for an unsilenced H or X pipe.
- Rear boxes - modify the internals to reduce the muffling effect. (Modifying GS boxes work well as you can still use the valves - if you have the pneumatic operating system fitted). Or, fit ‘straight throughs‘ which is basically a piece of pipe with 2 outlets on.
Changing only one of these elements will give a small increase, which might be enough for some, but changing 2 will give a much more noticeable increase. Do all 3 and you are looking at an ASBO if caught. That’s why I now have a 4 valve system on my back boxes in which 2 valves ‘fail’ open and 2 valves ‘fail’ closed. This set up gives a decently muffled quiet side or a barking mad fully loud side.