This ship class is a favourite of mine! Four were actually planned and all were to be named Invincible, Indefatigable, Indomitable and Inflexible, and I tell ya, if one of those survived to this day and armed with modern weaponry, it would be insanely powerful, it's possibly strong enough to take on the Iowa class head on.
Big, mean, armed beyond the teeth, definitely my kind of ship! XD
Although their armor protection would've been updated over various modernizations, their protection was actually at least equal to that of contemporary Battleships at the time (A hard lesson learned from the Battle of Jutland).
Yet despite their original classification, it wouldn't take many refits to reclassify them as Battleships, and even they were capable of taking on the German Battleship Bismarck, even though the G3's were built two decades earlier than the Bismarck. They would also make the Japanese Number 13 class Battleships their chew toys and even they were armed with larger naval guns of 460mm (18in) calibre.
Their 'true' Battleship cousins, the N3's, would've been absolute nightmares to deal with since they carried much thicker and heavier armor and even larger naval guns of 460mm in comparison to the smaller 406mm (16in) naval gun. An N3 and the Japanese Yamato were well matched in the size and number of the main armament (Nine 460mm naval guns within three three-gun turret housings), but the N3 would easily make Yamato think twice before engaging.
True, the Iowa class is redeemed as 'The Ultimate Battleship' in terms of design, but their protection was no better than the earlier South Dakota class Battleships, as the Iowas were merely lengthened versions of the South Dakotas. The Montana class would've had armor strong enough to shrug off their own shells (Mk8 AP Shells) while the Iowas armor would've given them protection from guns of up to 300mm (12in), which was planned on the Japanese Design B65 Cruisers (Developed to counter the US Alaska class Large Cruisers).
Dont forget that the last British battlecruisers still had crap armor. This is shown quite clearly with the case of the HMS Hood. I do have to agree about how the Montana would have been a world beater, but I guess we will never know. You also forget that the Yamato class may have been armed with 18.1 inch guns, but the Iowa's 16 inch guns had better penetration and did more overall damage.
HMS Hood was part of the predecessing Admiral class, which only Hood was completed with her three remaining sisters cancelled (mainly because the German Battlecruiser building program virtually grounded to a halt), and the G3's were to be the post-war product of a well-protected class of Battlecruisers, and if the Hood had been thoroughly rebuilt like HMS Renown, Warspite, Queen Elizabeth and Valiant, then she would've survived her ill-fated encounter with KMS Bismarck!
Even though the British Battlecruisers had relatively inferior armour, they were still well protected. For example the HMS Renown survived multiple hits from 11 inch shells fired from the German Battleships Scharnhorst and Gneisenau, and if it weren't for her rebuild then Renown would've been at the bottom of the North Sea.
I already know that the IJN Yamato wielded the largest naval guns in history, and that they were outclassed by the US Mk-7 16 inch gun, and there are a ton of facts I know about 20th century Naval history, so let me ask you this: Why question me about facts I already know about? I don't want to sound rude or anything but this sort of thing gets on my nerves every bloody time.
And I even said that the British N3 and the Japanese Yamato carried the similar size of armament earlier on, although I think you mis-read my earlier comment about that bit.