The thing is, those sloped 307mm were optimised against the plunging fire angles of longer ranges;
At long ranges, indeed, the combined angle of attack of the shell trajectory + the angling made up for an angle of up to 45º, in which case it would go up to 435mm of effectiveness from ranges where the shells would have lost up to 50% of their penetration power already, dropping them to values on the 440mm pen range.
However, in War Thunder, typical engagement ranges are actually <10km, which means the shell trajectories are essentially flat, making the angle of attack way lower, and the shells retain up to 66% of their penetration power.
For example, let’s say:
Mutsu’s No.5 APC shell vs Iowa, 20,000m;
No.5 APC shell penetration at 20,000m: 460mm
USS Iowa’s effective belt at 20,000m: 435mm + 40mm (external hull) = 475mm
Quite impressive! Iowa’s armor actually withstands Mutsu’s shell. Right?
Well… except that’s at 20,000m, the range Iowa was designed to fight from.
This is not the case in War Thunder. In War Thunder, they will face each other from 10,000m away… and this is what happens then:
No.5 APC shell penetration at 10,000m: 555mm
USS Iowa’s effective belt at 10,000m: 340mm + 30mm (external hull) = 370mm
So… while Iowa’s armor is nearly impenetrable at 20,000m, it just doesn’t work at War Thunder’s current engagement ranges. See what I mean?
Always remember; an armor being excellent in real life under the circumstances it was designed for doesn’t mean it would be as effective in War Thunder when it would be forced to function under circumstances it wasn’t designed for.