You also over estimate the effectiveness of the Bismarck on its own.
Apparently so, I just think 8 15 inch guns are going to be
mildly supieror to 6 destroyers ...
However, Britain was worried enough about it, look what they did to prevent it getting into the Atlantic. They moved everything they had (HMS Ark Royal was brought all the way from a mediterranean mission, which left a convoy almost completly unescorted. Britain therefore was more worried about the Bismarck then the entire Regia Marina.
How effective it would have been is leess of the issue, the main issue was the threat. The Bismarck could have very easily destroyed an entire convoy. Something a U-boat would have found impossible.
The United States sunk or severely damaged many Japanese battleships in world war two by totally insane and reckless destroyer captains.
British destoyers had less luck against capital ships though, different doctrines. Besides, unlike the USA, Britain didn't have any destroyers to waste, only when they were sinking (I.E HMS Glowworm) did they try this, but that didn't even sink a cruiser, let alone one of the most protected ships afloat.
The British fleet would not have taken long to totally destroy the entire Kriegsmarine surface fleet with aircraft and surface ships in a matter of months if they went out into the open sea.
Possibly so, but in those few months, most convoys would be suspended, or rerouted miles off. However, by 1941 this could be abit late, but the threat was very real.
My argument is simply saying that, at the start of the war, the surface raiding doctrine was too terrible, well, better then 300 Type II and VIIa's floating around.
edit:
The orders would be scatter whilst thye escort, close and distant, sacrificed itself in order to buy the convoy time. PQ17 is a great example of why a surface raider would have had a far more limited effect on its own than you suggest. But, and here is the big one, a surface raider can break apart a convoy making its constituent parts easy meat for subs and aircraft.
K
Convoy ships travel at around 9knts top speed, Bismarck was a little faster then that. They whole point is that it endangered convoys, convoys therefore had to be bigger, took more type to setup, required more destroyers, this took additional time. Of course, by 1941 this isn't as big an issue then in 1940 ...