The 343mm gun was fairly mediocre or average, in post war trials the 305mm Russian gun matched the penetration the RN gun attained using the improved Greenboy shells. That alone tells you a lot and lends weight to the German outlook that their guns were close to equivalent to the RN guns.
You mean "
much better than the new Greenboy British 12" Mk VII APC shell did and seemingly on a par with the larger British 13.5" APC shells" [
source, presumably; my emphasis added]. You've confused Nathan Okun's statement to refer to 13.5in Greenboys when in fact he's talking about the standard 13.5in shell. You've also changed the appearance of parity with the achievement of parity: the
test was only between 12in shells, and Okun doesn't say that the Russian 12in was
actually as good as the 13.5in gun, only
seemingly so. Finally, it's worth noting that that the Russian gun being referred to fires an exceptionally heavy shell, far heavier than those of British and German 12in guns (Greenboys or not):
British 12in/45 and 12in/50 - APC Mark VI (2crh) - 850 lbs
British 12in/45 and 12in/50 - APC Mark VIIa (Greenboy - 4crh) - 854 lbs
German 30.5cm/50 - APC L/3.4 - 894 lbs
Russian 12in/52 - APC mod 1911 - 1,038 lbs
And the 13.5in shells for comparison:
British 13.5in/45 (light) - APC Mark IIa - 1,266.5 lbs.
British 13.5in/45 (light) - APC Mark IVa (Greenboy) - 1,257 lbs.
British 13.5in/45 (heavy) - APC Mark Ia - 1,400 lbs.
British 13.5in/45 (heavy) - APC Mark IIIa (Greenboy) - 1,410 lbs.
I'm not quiet clear why you think the 13.5in guns were mediocre, though. As it happens, particularly with the heavier shells, they didn't compare too poorly with larger guns of roughly the same type:
French 34cm/45 - APC M1912 - 1,224 lbs.
German 35cm/45 - APC L/3.6 - 1,323 lbs.
Japanese 14in/45 - APC Type 3 - c.1,400 lbs.
US 14in/45 and 14in/50- AP Mark 8 Mods 3, 7, 8 and 11 - 1,402 lbs.
Russian 14in/52 - APC Mark Ia - 1,586 lbs.
Even if the RN shells had worked as intended, they didnt have a long delay, they were intended to destroy plate by detonating as they penetrated, thus making a bigger hole. Delayed German shells would still have been far deadlier.
This comes back to doctrine, though, and misses the point of what I was saying. If you're facing your opponent and have the choice between 1) an inch less of armour and an inch more of gun calibre or 2) an inch more of armour and an inch less of gun calibre, you should always take option 1). Option 1) gives you the same chance of penetrating the opponent, but a much higher chance of doing damage to them via bursting charge. The Germans undoubtedly knew this at the time, which makes the question of why they chose inferiority is an interesting one.
The 50 calibers 305mm RN gun was a failure, so in order to get the required performance they went up in caliber.
From /45 to /50 is still a step up, and still done a year before the Germans make a step up from 11in to 12in.