Determining the strength of a gun doesn't depend on the nature of the load but the nature of the gun. Simple mechanics, (Force=Pressure(Area)), tells us that a gun's maximum "power" is as simple as multiplying it's maximum allowable pressure by the area of the bore.
For a 12-ga shotgun, the cross sectional ares is: π(0.729/2)^2, or 1.15-si
The SAAMI maximum pressure for a 2-2/4 or 3-inch 12-ga is 11,500-psi. So its F(max) is 1.15(11,500) or 13,225-pounds
A 3-1/2" 12-ga has a max pressure of 14,000psi. So its F(max) is 1.15(14,000) or 16,100-pounds. IINM, in the UK, the max allow pressure is 15,000-psi which would yield a F(max) of 17,240-pounds.
Out of curiosity, let's have a look at a .30-'06 Springfield cartridge: rather than calculate the cross-sectional area of the bore and grooves, I'll use SAAMI's area of 0.0737-si with a pressure of 60,000-psi. The 30-06's F(max) is 0.0737(60,000) or 4,422-pounds. Significantly less than a 12-ga shotgun.
These are a scalar quantities of the maximum allowable forces. How the gun performs within the max pressure limits is up to the cartridge loader not the gun maker.
how powerful is a 12 gauge loaded with high brass buckshot or slugs?
Depends on whats inside the shell High brass can mean its a higher power load, or it could just mean the manufacturer decided to use high brass on their wussy birdshot loads.
Sirslope is correct. At one time, in the days of paper shells, high and low base shells were an indication if the shell's power (not the gun's). With modern plastic shells (some of which have no brass), the bass height is just for tradition and identification purposes.