I tested the Ki-109, Ki-102 Otsu, Ki-45 Toryu, and J5N1 Tenrai (Japanese heavy interceptors) against the B-17.
Some of these matchups would not normally happen in regular battles because of BR differences, so this was done in a controlled custom mission to compare damage behavior.
My conclusion is:
The 75 mm cannon can still blow the B-17 apart with a lucky hit.
The 57 mm cannon is already much less reliable and requires very accurate placement.
With 37 mm and 30 mm cannons, simply scoring hits is usually not enough. In many cases, you need to keep concentrating fire on critical areas, or set multiple engines on fire, before the B-17 finally goes down.
And of course, in a real fight, you would need to maintain that level of accuracy while being exposed to the bomber’s defensive gunfire.
Conclusion: even with Japanese heavy interceptors, big guns alone are not enough. You still need very precise aim, and doing that while the bomber is shooting back is another problem entirely.