Check out some of Murray Carters videos.
If you use the entire stone surface, overall wear will be very even and you'll only loose a small amount of stone when lapping it flat. If you fail to do this and concentrate wear in a few areas you'll have to lap more often and will loose quite a bit more of the useable material. I took a diamond sawsall blade and cut a 1/4" off the end of all my waterstones (Kings and Nortons) and use it to whip up polishing mud, and to give a quick scrub when I finish - keeps the surface reasonably flat indefinitely.
I'm not sure about using higher pressure on these stones, I'd keep it light.
Edit to add:
Grind time on most steels will be very comparable, you'll only notice a difference on the high alloy and high carbide stuff. Even higher RC carbon steels will grind pretty quickly.
For another idea of how to use all the stone's surface you might check out some of my Washboard videos (through the link in my signature). Is not a waterstone, but in use I work it the exact same way, substitute a splash of water for every time I stop to unload the paper with an eraser - I actually think of it as being something of a ghetto waterstone in terms of application and how it 'feels'.