The new leather I use is 7mm thick. The 'Professional' model leather is only 5mm.
Yet an hour is sufficient for my vacuum forming.

I doubt that Brooks would use a water soluble dye for a saddle. But I don't know.
If you look at the videos of the making of the saddles, there is no evidence of each cut being inspected midway through the immersion.
And with the massive presses they use, there is no need. More marketing spin.
Partial immersion will lead to a shore line, as I have written before. This is not normally sought after.
I have the advantage of working with new leather where the leather hasn't been filled with unknown treatments.