So much bad information here. All horsepower reported in the US, unless designated otherwise are standardized SAE units (for example, in Germany, the hp is reported by German engineering standards, DIN, whereas US uses Society of American Engineer standards, SAE; thus the BMW in Germany reports 343 DIN hp while in US it's 333 SAE hp). Thus, there is no German hp is better than the Japanese hp if report comes in US.
Now, why do BMWs seem to have so much hp? The reason lies in the gearing. My father's 330i pulls massively in 5th gear, surprisingly fast. The reason is the gear ratio. The BMW gear ratios are better suited to the engine they work with (as oppose to our WRXs with the frigging 4th gear acting like it's another overdrive). 5th gear in 330is and 330cis are 1:1 ratio (this isn't even remotely close to an overdrive ratio!). That's shorter than our 4th gear I believe. Thus, the correct phrase is "those Germans know how to utilize their hp better". However, due to that 1:1 ratio, BMWs aren't getting as good of a gas mileage as they can get.
So when car's have better "top end" acceleration, you have to look at several things. One is the wheelhp to weight ratio. Another is whether a car has initial hole advantage shut as AWD. Gearing is also rather important. For ultimate top speed, aerodynamics and total crank hp comes into play.
Of course, there's always underrating the car like the Dodge SRT-4, however underrating the engine as much as Dodge did with that car is rare. If anything, underrating would come only 1 to 5 hp.