Original brake discs were chromed for the showroom and practically useless, especially in the wet. When Commandos were new we took the chrome off and had a brake after that (of sorts- average for the times). Today's Andover Norton brake discs are coated with the stuff automobile manufacturers use, so where the pads wear this goes off imnmediately and you got a brake.
I found drilling the discs useless when we did it in the 1970s, never tried it again bar on our race bike (Production Racer double disc setup), there primarily for weight reasons. On production brakes it gave us no more braking, not even in the wet, but a lot more wear on the pads, even though the bores were deburred. Wouldn't do it again.
Joe Seifert