How loud will a cat delete be on an audi a4?

This is NOT an OLD CHEVY CAMARO here! The AUDFI is made to HGIHEST specifications and the removal of the CAT CONVERTER will only RUIN the power and the MILEAGE! If you want MORE POWER< use DENSO TWIN TIP IRIDIUM LASER spark plugs! IRIDIUM< I call the DINOSAUR element< will burn MORE fuel MORE completely! A&UDI requires ONLY 502/505 oil AND never VALVOLINE EVER! The use of PENNZOIL PLATINUM will get you a 500,000 miles WARRANTY!

Be prepared to get your car impounded.

Not much louder.

Just going to need to buy a ~$600 tuner and dump another $400-$1000 into having it tuned to run properly.

Might hardly make a difference in sound, but the performance will suffer.

Catalytic converters don't effect the sound. You need a cat-back exhaust system. The muffler is what makes it sound different.

Check out a Borla system.

You're wasting your time because it is against Federal Law and it won't give you better performance. 

It is illegal to tamper with the emissions system.  Be prepared to get your car impounded.

It'll sound like there's a hole in the exhaust system and it will run like shīt unless you have a way to fool the lambda sensor input. Even with one it will still, be a little down on power as the catalytic converter adds resonant back pressure in addition which gives a very mild supercharging effect. The silencer (muffler) also does that, but with the cat removed the resonance to the silencer changes and larger ruins the effect.

The car will also be illegal.

If you want a sportier sound then leave the catalytic converter alone and fit an aftermarket silencer.