Avoiding the details of the calculation and assuming standard pressure and temp with a 34 mm restrictor the max mass flow is about 0.22 kg/s; let’s say A/F = 14 hence 0.22 / 14 = 15.7 10–3 kg/s fuel.
Energy per unit mass of fuel (the heating value) is about 47-48 MJ, and the engine efficiency is usually about 32-33%, hence as first order approximation we can reasonably hope to extract something in the order of 15.7 * 48 * 0.33 = 248 kW, make it 250 kW = 335 hp.
Based on that calculation and given that very likely max power isn’t the main concern in the application, I would say that probably best engines are between 320 and 330 hp.
But I would also add that there’s probably a not small difference between the best ones and the others, both on peak power and, mainly, in power distribution on the rpm range; at the recent Monza Rally Rossi was driving an official Prodrive Impreza and even from the grandstand the engine difference with the Fabia driven by Capello and McRae, official too, was very evident, that blue car is simply a rocket.
And that without considering the difference with the older and private cars, I’ve seen Rossi passing a couple of 2003 206 WRC in less than 200 m after a slow speed corner, and notice that Rossi was visibly slower in the corner and opened gas later... God knows what that Impreza would have done in the hands of a good driver. Certainly it wouldn’t have been beaten by Capello’s Fabia.
Anyway the really amazing thing of the WRC cars is that even with such “low” power, they would leave easily any 500 hp Group B way behind in just few hundreds meters of a stage... It’s when you look at these things that you see the incredible difference 20 years made.
If you look at the power curve of a WRC car they hit 300BHp at about 2K RPM,
That would require torque to be more than 1000 Nm, and clearly it can’t be. To find an engine with a torque of 1000+ Nm you have to look at the new Audi R10, and that’s a 5.5l twin turbo diesel.