- RTA won't be the one pulling you over, it'll likely be HWP or general duty police.
- If they see a DCI you could get a defect which you'll have to get cleared by someone on the rta's list of registered mechanics. When you put your stock airbox on, the mechanic will see the DCI has been removed and most likely give it a tick and you're off on your way. Unlikely (time/effort constraints) the mechanic will then go inspect everything on your car.
If for some reason you ever got a full shakedown and had to report to the pits for a thorough vehicle inspection then I would put the stock cats in... for $500 install, consider it 'insurance' against a potentially much larger fine.
Now away from all these scary stories, who here has ever been defected in a BMW OR been asked to open their bonnet?