I will have to disagree Lane.
The Honda engine may not have been the most powerful but in the Super Aguri (another weak chassis) it showed enough grunt to overtake some of the big boys. The Mercedes engine is probably superior but to shoehorn it into the chassis so late in the development curve of the car (Jan '09) surely there were compromises made so that it may have no net benefit over the engine the car was originally designed around. IE. You MAY gain a few horsepower, but how are the aero and the suspension of the car affected? I'm not an engineer, just thinking out loud.
Bottom line, don't attribute the Brawn pace to the engine, the car has a great design and a great rear diffuser.
