Hi Walruss,
If you'd like to try something REALLY risky, try to re-create that effort on a high realism setting. At the lowest settings you can get into gun battles with escorts and survive, or even emerge victorious as you did.
At the higher levels, however, you'd be fish food within minutes, if not earlier. My guess is that you wouldn't try it more than once.
The logic of this is simple: In a sub, you're firing on an unstable platform with very little protection. The gun crew is essentially defenseless. Your deck gun is only a single barreled crew-served weapon.
In contrast, the escorts have a much more stable platform, more often than not they have armor protection, they have multiple weapons with multiple barrels. Add in radar guidance, faster reloading and much, much bigger calibers and what you have is a recipe for disaster for the unwary U-boat Kaleun.
The higher realism setting are much more of a challenge, but if you get into the game, you'll find much greater rewards. Without the option to engage in a gun battle, when taking the time to develop a solution on a rapidly approaching escort is consummate to suicide, you'll find the thrill of switching roles from the hunter to the hunted. You'll have to be crafty and cautious to survive, but will have a much more enjoyable experience all the while.
Give it a whirl. Good luck and happy hunting!