On searching for "steam locomotive power formula" I found - among others -
http://www.smex.net.au/reference/TractiveEffort02.php - which gives
and also has a power (and tractive effort) calculator.
Putting various classes into this gives - at 60 mph, with K=0.85, and in no particular order - the following: 9F 2-10-0 6350 hp; GWR King 6450 hp; LMS Duchess 6400 hp; LNER (Peppercorn) A1 5980 hp; A2 (Blue Peter) 6470 hp; P2 (Gresley as built with 220psi boiler) 6950 hp; P2 'Prince of Wales' (new build with 250 psi boiler) 7900 hp; Gresley proposed (1939) 4-8-2 7310 hp; BR Duke of Gloucester 6250 hp; Hypothetical design with 3 21"*28" cylinders and 6'-2" wheels 8510 hp @ 250psi, 9530 hp @ 280 psi (exceeding anything built, but practical max for Britain? taking max figures from different designs; freight version with 5' wheels 10500/11800 hp respectively, to indicate what might have been achievable).
But all these are very much theoretical figures for rough comparison purposes only: in practice would be affected and restricted by the ability of the boiler to generate enough steam for long enough (and crew ability to get it do so), the valves' ability to get the steam in and out of the cylinders, the adhesion to get it through the wheel/rail interface, etc, etc. So large spoonful of salt required...
But they do indicate that the P2 Trust claim is not without justification.