You could always contact McLaren yourself to confirm. Outside of that, do you think you can stomach the risk? If yes it's not a biggie then go for it. If you think you really still want to keep the warranty, then don't do it. Extremely simple.
In the US, they are protected by a court ruling, Magnuson-Moss Act. Not sure if the same thing can be apply to Canada. The dealership and McLaren has to prove your aftermarket addition is the cause of the problem before they can void the warranty. The Act split warranty into 2 parts, defect and performance on emission parts. i.e. pretty much the whole engine assembly and accessories. The defect part will be void as you have replaced/removed the cat, a important emission part. The performance part however cannot be void as McLaren will be required to bring the car back up to the emission performance standard that they promised even with you aftermarket part. But the end result is that McLaren will throw away your cat-less exhaust and put in a factory one to fix your car, as that would be the only way to bring the car back to the factory emission performance level.
But the good news is, even if McLaren void your drivetrain warranty, they cannot void your electric seat warranty as the exhaust will have nothing to do with messing up your seat.
