16 3/8" to 16 5/16" is about the max overall tub depth I've seen. But what you really need to know is the distance from the bottom of the tub to the bottom of the overflow drain. That is a better way to determine maximum depth. Additionally, American Standard offers a deep soak drain (I have an extra if you're interested) which raises the height of the bottom of the overflow drain an additional ~2" regardless of where the bottom is carved out of the tub. So no matter what brand of tub you chose, if you used the AS deep soak drain/overflow system, you would increase your maximum water depth by another ~2"!

Toto's FBY1525RP (12.75" water depth), American Standard's Spectra (11" water depth) and Kohler's Bellwether (9" water depth) & Guardian (10.88" water depth) all fit the bill as being made of cast iron & height of tub/depth is greater than that of the Kohler Villager (which comes in at 14" high w/ 8.63" water depth). If you could do a drop in tub and use a different material for the front, that would open up more options like Kohler's Tea for Two (13.63" water depth) & Parity (13.875" water depth), Porcher Nouveau, Toto's FBY1550P.

Hope this helps! Let me know if you're interested in the drain!

Kohler's mendota and memoirs are both a little deeper than the Bellwether, but only a little. If you don't mind ugly drain hardware, AS has a drain that has the hole in the overflow at the top, so you can get a couple of inches more depth from any tub, but it has a large American Standard logo on it. (Kohler has a much more attractive overflow, but it's only for certain of their models.)

We have two Toto 1525's, and like them very much. If DH or I want to soak, we can't lie down and fully stretch out in the tubs (I'm 5'3" and DH is 5'8 1/2"), but our bathrooms wouldn't allow longer than 60" tubs.

Despite this, we still find them to be very comfortable tubs.