Good questions:
I did not soak it prior to lapping. It seems odd to me that even the honing directions advocate soaking that stone for like 30 minutes. It does not seem like a porous stone to me.
You're correct, the middle was much higher than the ends which made for why it took so long (had to remove most of middle to even it out).
It did produce slurry during the lapping and I kept the stone/plate plenty wet while lapping. Still, it's the hardest natural stone I own, much harder than my yellow coticule, belgian blue, Tam O' Shanter.
Chris L