The guys on castboolits can answer better than I. I am relatively new to the casting club. However, based on my reading, that should be hard enough for decent penetration. That is one of the things Keith was after with his bullet designs, and he was using a BHN of 12 as mentioned above.
I live in the Rockies, and load currently .401 175 gr LSWC under Blue Dot or AA#9 in 10mm for bear repellent. My alloy air-cools to a BHN of 11-12, but I drop them from the mold into water to get a very hard 27-30 bhn. I have not yet performed penetration tests...but they are accurate and should penetrate very well, even at a bhn of 12.
Once again, check in at cast boolits. My experience is limited to one caliber. I have devoured lots of literature, so I have 'book smarts' on the subject, but guys there will have actual experience too.
Edit: If you want them harder, heat them in the oven for one hour at 400°. Then remove them and immediately quench them in water. They should jump up to a bhn of 25 or so. Best to heat them on a pan with holes, cause they will be soft when they come out. Bumping will deform them. And, you will have to re-lube them with something like Lee Liquid Alox, as the lube will melt out.