You are correct, I had remembered the details wrong- ISS and Mir and Salyut 7 all have the same orbital inclination, so that's clearly the best orbit for Baikonur. So if you were doing your mass launches from Baikonur you would not pay much of a penalty. From KSC, though, you'd almost certainly want a lower inclination to get more payload up.
Really, you pay the same penalty launching to 51.6 degrees regardless of whether it's Baikonur or KSC. But you're totally right, you can do even better by launching to lower inclinations out of KSC, which can go down to 28.6 degrees.