Heh Kakemann - my modding skills are sadly not as developed as yours. I hold your work in great respect. What i can do is deliver what small knowledge i have from reading a few books. To make that happen in a mod would take "other people"
Regarding going to 460 ft. - the thickskinned fleetboats (starting with Tang in 1944) could go to about 600 ft. The depth guage showed up to 600 ft. and the pin was at 612 ft. At one point around the Marianas Tang took seawater in through a torpedotube in the forward torpedoroom. the imballance made the boat go down through 612 ft. for a while. They had no idea if they were then decending or rising. After a while it started moving back towords 600 ft.
Tang went to 500 ft. many times to duck under layers.
From reading the forums i come to belive that the Layer depth are hardcoded in some way, so we can not make them be deeper.