not sure how much this helps you, but I use Evan82's XXI mod which includes his type XXI flak mod. As designed, the XXI was to have 30mm MK 303 cannons firing a 30×210mm cartridge, but the guns weren't ready in time so 20mm guns were used instead. The mod makes the guns a good deal more powerful. I've found they still don't effortlessly destroy patrol bombers, but they make short work of Avengers and they're very effective (a little too effective) against small ships. I frequently use the guns to attack merchants of less than 3000 tons and it often takes less than one belt to do so, especially if you're putting most of the hits below the waterline.
It's definitely not the same as an actual deck gun. You have to fire it manually of course, and the effective range is a little shorter unless you're in very calm seas. But as a frequent Type XXI driver, I find the guns more than adequate against typical deck gun targets.
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