the problem with using HP to compare steam locomotives is you need to measure the ability to transfer that HP to the rails through tractive effort. In this case, the PRR kinda comes short in their claims.
Q2 tractive effort =
100,816 lbf (without booster) / 115,816 lbf (with booster)
UP Big Boy tractive effort = 135,375 lbf (602.18 kN)
C&O Allegheny tractive effort = 110,211 lbf (490.24 kN)
N&W Y6b tractive effort = Simple: 166,000 lbf (738.4 kN)) (Compound: 126,838 lbf (564.2 kN)) (as built 152,206 lbf (677.0 kN))
The Northern Pacific fielded the Yellowstone class 2-8-8-4’s (B&O called them EM-1’s) and had a tractive effort of 115,000 lbf (510 kN).
that said, the Q’s were beasts for sure!