Ontario is different for this type of thing since insurers have more discretion to deny or surcharge vehicles based on modifications and will cancel you if they get a whiff of motorsports.
The person at the autopac location likely knows very little about this and I'd be willing to bet the person they spoke to at MPI was a CSR or something similar. A MPI claims person would likely be able to shed more light on how they would handle this situation, but I'm not sure they are available to speak to with questions.
The area of the vehicle inspection book that might make reference to this is on page 31, section 10-7 - Frame and/or Structural Body Components. It states "(B) inspect for evidence of collision damage or additional welding on frame rails or unibody" and to reject (fail) if "(D) structural components does not meet OEM or I-CAR standards"
MPI/rules aside, a cage is a bad idea for a street car. Higher costs to repair and potential to cause additional damage to the vehicle itself. Potential increases to both frequency and severity of injury, not to mention potential injury to passengers who may or may not be aware what a cage can do if you aren't wearing a helmet. Plus you are compromising the safety design of a vehicle, especially if it's newer and didn't come from the factory with a cage like the GT3RS.
Calling Vehicle Standards/MPI for their official answer is likely the only way to confirm how they would respond.