Actually for the QE class the war college wanted a fast battleship wing for tactical reasond to turn the enemy's van so a min 25 knots was required for this. In the US they had just launched US Texas with 10 14" guns firing a 1500 pound projectile (I was fortunate to go aboard her last month) so a broaside weight of 15,000 pounds. Iron Duke fired the 1,400 shell so broadside weight 14,000 pounds. Canada I think also fired 1,500 pounds. Agincourt 12" shell was 850 x 14 = 11,900 pounds so well below Iron Duke. QE shell weight was 1,920 pounds x 8 =15,360 pounds. The 8 gun layout led to a more logical design and as mentioned more boilers and higher speed withou sacricising armour.
They were a special class which is why the subsequent R class were a little smaller but a lot better protected but retained the 8 x 15" guns as broadside weight was deemed sufficient. Also 8 guns made for more logical salvo firing.
Geoff