If you have to tap, you have a vertical rotary spindle of some sort, so it just makes sense to use that same spindle to drill as well. It's not like you could, or would, hand tap all those holes. Use a spiral pointed tap to force the chip ahead of the tap and you never have to back it out to clear it. Typically gas cut and plasma cut edges get harder than the surrounding metal, even when they are mild steel, and a plasma cut hole isn't likely to be too round since you have to pierce the steel to start, with the plasma anyway.