Remember, apart from th added attention from Police etc for excessive noise, any (ANY) modification(s), unless approved to ADR standards, that change/alter/modify any aspect of your vehicle from the ADR standards/requirements that existed at the time of manufacture of your vehicle are ILLEGAL.
Benboy is right with what he typed, but he's just emphasing the points that are important to his argument. I'll state the same facts, but in an alternative fashion:
It is PERFECTLY ACCEPTABLE to make any (ANY) modification(s) that change/modify/alter your vehicle standards/requirements that existed at the time of manufacture of your vehicle, provided you stick to ADR standards AND ANY ADDITIONAL REGISTRATION REQUIREMENTS FOR YOUR STATE, and it is certainly NOT ILLEGAL for you to do so. In meeting these standards, you will not receive unwanted attention from Police etc. for excessive noise.
See
. We've stated virtually the same facts, but it sounds completely different!
In regards to exhausts, ADR standards dictate limits to Noise and Emissions.
The proposed parts come from the same class of vehicle, in fact almost identical... except for the obvious engine and drivetrain differences. A V8 from the SS is going to produce more noise and emissions than the V6 in the Omega. Accordingly, because the standard V8 exhaust components satisfy all ADR's when used on the V8 from new, they will continue to satisfy the ADR's when used on the V6.
It is also why it's perfectly legal to buy and fit some sports exhausts to most cars. Provided you stick to the ADR limit (which is 79dB-80dB depending on standard Kw produced by car), it is perfectly acceptable to increase the noise from your vehicle up to these limits.
Of course, there are a lot of people out there who don't follow the rules... but that doesnt mean that all non standard exhausts are illegal. Far from it.
And to the OP, the rear muffler dictates most of the exhaust note on the car. Instead of going for a rear muffler designd for a V8, I would suggest going for a sports muffler designed specifically for the smaller capacity V6, otherwise you may be left with an exhaust with the dreaded "V6 drone" that most people try to avoid.
It's also why I wouldnt fit the full V8 exhaust to the V6. You'll lose backpressure, lower the velocity and temperature of the air escaping, and as a consequence LOSE power. Bigger is not always better in exhausts.
Using a little logic, the V8 is going to produce more gas and in a different pattern than the V6. Getting a muffler that is more suited will give you a better sound, and keeping the same exhaust components 'up stream' means you wont cause any issues with performance.
