There are a couple ways to get some idea of how much stronger the BH is. For one, Ruger puts 7 of the new .327 mag load in the GP and 8 in the BH.
The second is just simple dimensions. I just took some quick measurements of the cylinders. There is quite a difference. For the diameter of the cylinder I get 1.545in for the GP and 1.731 for the BH.
The web between the cylinders is 0.15 and 0.19 for the GP and BH respectively.
For the outside wall the numbers are 0.1in for GP and 0.135in for BH.
So the sheer size difference indicates a stronger BH, but of course it would be more complicated to actually determine how much stronger.
I'm glad that I finally did those measurements, I never really had a good handle on the dimensions before.