Yeah, that's kind the point; with three different lines to detect incoming Soviet bombers, any lackings in SAGE itself is made up for by the fact ADC has enough assets and warning time to respond. Would it be perfect? Of course not, but more than sufficient to prevent any serious damage. I'd also say comparing the North Warning System failing to detect a single bomber is rather different than dozens if not hundreds of bombers crossing three different warning lines set up on a wartime footing.
I'm not sure how useful the Mid Canada line would have been. It was a rather different type of system than the DEW line or the pinetree line and didn't stay in service very long.
There were also concerns about the 1950's era GCI radars being jammed. I seem to recall there was a subsequent effort to deploy "frequency diverse" radars to mitigate that issue. I don't recall the exact dates but I suspect the jamming concerns would have been valid in 1957.
(Edit to add:
My understanding is that in general the radars in use in 1957 were better than the world war 2 era radars that the U.S. initially deployed. Also in my view the wide spread deployment of jet powered interceptors with their own air interception radar was a major accomplishment by the U.S. and the Canadians in this era (as was the construction of multiple radar chains, the deployment by the U.S. and later by the Canadians of nuclear armed air defence weapons etc..)
That being said NORAD presumably had some
immediate work arounds re the potential jamming issue(s) and given the sheer numbers of interceptors they had and the small numbers of Soviet bombers I expect NORAD (and the SAM's and guns if any were still in service in the CONUS in 1957) would have done reasonably well, but I suspect a few bombers would have made it to their targets.