I've shot several fighters down with just the .303s, including a Bf110G-2 that I took the wing off of.
Most that believe dumping the .303s makes a huge difference, problem is you actually have to need ACM to make the difference count.
I really wish (and it was requested many years ago) for an ammo slider so you can decide if you want 100% of ammo or 0% - I would certainly
load 100% in 20s and maybe 50% in the 303s.
And I agree, last sortie I flew in a mossy I ended up with 4 kills all were 303s when I strafed down all my 20s on a town.
It does take precision aim and most don't have that level of skill, hence why most want that 1 shot 30mm power. What most people simply do is realize "it won't down a plane" well the key is not to bring the entire plane down in 1 pass, but if you use 303s to blow a rudder or flap off, it makes it that much harder to avoid a mossy, and this is what I generally try to do on a snapshot.
If I could have one prototype in game, I'd love to see the 4x 303s replaced with 4x browning 50s in a mossy. (This prototype was just an idea and never actually created however).