HP usually has more water than CP simply for processing -- it has to be fluid enough so you can stir it for best results.
The rate of saponification is partly related to the concentration of the lye, yes, but the overall temperature of the batter, the type of fatty acids in the recipe, the degree of mixing, and other issues affect the reaction rate too. I honestly doubt you're going to cut the saponification time in half just by reducing the lye concentration.
Honestly my shave soap doesn't need an hour long cook even at a relatively low 25% lye concentration ... 15 minutes is all it really needs ... and I can mix it if I use a sturdy spatula and some elbow grease. I'd hate to see what the batter would be like to mix if I used a 50% lye concentration so the batter had only one third the water!