
My estimate is that they would have devastated bomber formations in the first wave of an intercept mission; whatever got through them would have dealt with the other intercepters or SAMs over Canada and the US, or a second wave of Blackbirds.
Against a conventional strike package, I'm not sure how they would have faired. The best bet would be to stay VERY low and hope that ground clutter affected their ability to track/lock/fire/home on you. A few jammer aircraft would help, but the YF-12s massive radar would have proven difficult to jam, not sure about the missiles, but home-on-jam defeats that whole tactic. The thing to NOT do is try to climb and attack them. In clear air, below and ahead of them you'd be missile fodder.
Then again try not attacking a country who can actually design and build a functional MACH 3+ interceptor that could carry neuclear tipped anything.... If the US had pursued that warhead option a pair of YF-12s could destroy your strike package by putting missile or two anywhere within a mile of it. Like playing horseshoes, 'close counts'
It would go something like this:
Eat breakfast with the wife/kids, go to work, fly out to your CAP 2000nm away near the 'unfriendly guys', vector towards a strike package as AWACs detects aircraft massing as they launch, intercept and blast 30 aircraft with a single 2Kt tipped AIM-47 75nm out (over their houses), fly back to hit the tanker, fly home at the end of your shift, land that afternoon, help the kids with their homework, then have a little fun with the wife.... after all you're a 6 time ace with a single shot!

Keep 'em flyin'

TEG