To be honest with you, if it's done properly and painted over, I don't even believe the random inspection mechanic will know the difference between the C-noteched frame and OEM (cause obviously he won't have one to compare it to)
Otherwise I wouldn't worry about the strength of that, if the C or U-channel that's welded in is welded in properly and is at least 1/4inch thick, the frame will probably be stronger in that area than OEM.
Il est interdit de toucher au frame d'un véhicule à moins d'avoir l'autorisation de l'ingénieur de la SAAQ.
Maintenant, reste à voir si le mécanicien est en mesure de valider si le frame sortait comme ça à l'origine ou pas.