Ooh, an 18-inch-gun Iowa...me likey!
There actually were several designs in the preliminary stages of the Iowa-class concepts.
those super heavy shells Iowa used had similar effect to Japanese 18" guns.
Misconception. It was still noticeably inferior to 46 cm.
The Type 1 AP shell actually had some astounding results with the deck penetration. The flatter cap allowed it to normalize against extreme slopes, but at a cost. It would penetrate that amount, but anything below 15km when it did penetrate generally caused the shell not to fuse due to how it penetrated. Much like how the shell busted through the turret roof of the Dunkerque from the HMS Hood, the shell would normalize, grip, and penetrate but due to the extreme pressure of the side gripping of the AP cap, it would force the cap into the shell body partially destroying itself in the process. So you get full penetration but with large fragmentation and after effects without fusing. Beyond 15km this is reduced and by 20km range it isn't nearly an issue. This issue corresponds with Japanese testing at angles of 10-20-30 degrees for the Type 91/Type 1 46cm shell and their post design testing to attempt to reduce the problem with new cap hardening and body tempering. Impacts under 15 degrees still suffered the issue but less so and by 25 degrees it was no longer a problem. Normalization for the shell was between 3-8% degrees. This is taken into consideration on range.
Overall, the Mark 8 AP shell starts to catch up to the Type 1 shell around 20-25km, and they normalize before the 46cm over takes it again. So at average combat ranges there is only about a 10-11% gain in penetration of the 46cm shell, but the larger bursting charge and direct MJ of kinetic impact goes to the 46cm shell. That isn't to say that the 16'' Mark 8 is weak, just physics do come into play.
Also, for future reference, don't use combined fleets for any information. They are extremely bias and very much poorly dated and understood by an Amateur who knows little in regards to Metallurgy, Physics, nor Naval Design and should be seen as for what it is, a hobbyist's perspective.
The writer has naval engineering training, so he's trustworthy.