The what-if I wonder about, actually, is-- suppose Semmes had gambled and won, and sunk or otherwise disabled Kearsarge? Where could he have gone, in a ship presumably even more damaged than she already was when she sought refuge in Cherbourg? She might have reached Le Havre or Calais, perhaps-- but her location was known, and Federal warships would (and did) rapidly converge from all directions. His only other option was Britain, where the ship would most likely be interned, or at least severely delayed.
Perhaps Semmes really did mean to go out in a blaze of glory, hoping to take a few Federals with him...