They can
You can walk into the dealership and order an R8 v10 engine if you like, they will give you it. The Decat pipes are not applicable to your cars chassis number, Audi can
only fit parts/accessories to your car that are applicable to your VIN. They can
supply whatever you want though
Warranty doesn't really work like that, you can't just fit OEM RS3 ceramic brakes and expect Audi to foot the bill if something goes wrong, when you pay for an option at factory order time the liability for warranty is included in the option price etc. This is why factory speed delimiter costs so much
The decat pipes yes are made for an RS3 but the bottom line is if they did not come fitted with the car consider it a modification and your warranty for the drivetrain null and void. You pay to play.
At the end of the day these Decat pipes are not a genuine dealer accessory, they are parts off of an RS3 designed for X market of car, not UK market. If an Audi dealer was to agree to fit these pipes they'd be snookered if they were scrutinised as they are technically committing fraud by increasing the emissions of the car, and we all know how safe VAG are playing it just now don't we
The two secondary catalysts are not "noise suppressors" or anything like that, they are 300 cell catalysts each. Their purpose is to reduce the emissions of the car. Trust me, there are far cheaper ways to suppress the noise.
What you say is factually correct, yes.
In reality though, chances are these Audi stamped bypass pipes will be completely fine without affecting your warranty. Just think about it logically for a minute, you take your car into your dealership with a leaking engine coolant hose or a dodgy alternator, they are going to fix your car and not even look at your exhaust setup. Even if they did, chances are (again) that the mechanic working on your car isn't going to notice the bypass pipes. Even if he thought something was amiss at that particular section of the exhaust, he would wander off, have a look, see the 4 rings stamped into the bypass pipes and carry on changing the alternator. He does not know the exact factory setup on the hundreds of different Audi models and variants that are up on his ramps.
There is a lot of unnecessary scaremongering on car forum sites. Although what you say is probably factually correct, the reality is, 99.9% of dealers wouldn't bat an eyelid at bypass pipes. You would also have to question the legalities of having say a Mechatronic unit failure on your S-Tronic box and having bypass pipes fitted being the reason for a rejected Mechatronic replacement. Even with engine failure, the bypass pipes are downstream of the engine and in no way effects the running of the engine. Emissions perhaps, engine operation, no. That's why chances are 'you'll be fine'. In the event of an engine failure anyway, it would take 15mins to unbolt and bolt the standard pipes back in if you were really concerned.
How about fitting RS wheels to an S or vice versa, surely the same logic would apply to warranty claims? You would forego all drivetrain and suspension warranty, brakes, driveshafts, transmission, shocks etc as it never left the factory like that. RS wheels may be heavier, they may be bigger. How about brake pads or tyres? You could fit stickier tyres that provides more grip. More grip = more stress on components.
Where do you stop with it? A bit of common sense needs to be applied.
Again, yes, what you say reference the secondary cat bypass pipes being actual catalysts is true yes. However, they are in effect just 'noise suppressors' to the customer. Removing them does not effect emissions enough for you to fail an MOT, removing them does not offer a performance benefit, removing them does increase exhaust noise volume though.
Personally, I'll see what happens when I get my car. From the videos, I think the facelift exhaust setup sounds perfect from factory. I'm 33 now, some may see that as 'old' I still think I'm fairly young but I've grown up and now past the stage of thinking overly loud exhausts are 'cool'. A video I posted up a few weeks ago of an RS3 sounding like a machine gun through a tunnel was utterly ridiculous, it sounded like a loud chav car to me. At 23, I probably thought that was 'cool' fast forward 10yrs, I just think the owner of that car is a bit of a loud show off. If it's not loud enough or needs that little bit more to give it a bit more depth/character then these bypass pipes will probably do the trick.
I'll probably go down the TTE625 path at some point in the next 12 months but having a screaming loud exhaust, nope, not a chance. The car needs to keep an element of class and sophistication. It's actually embarrassing dropping the kids off at school or dropping the missus off at work and turning up like a loud boyracer, everyone just stops, stares and thinks "di*khead". Exhausts can cross a fine line in my opinion, it's a tarted up A3 not a Lamborghini Huracan at the end of the day so be sensible.
Good point, leave all you slow coaches behind 
