The car with the most horsepower isn't always the fastest
So horsepower is just a number, and that number does tell you a lot about how fast a car can accelerate. But that is just one slice of the pie. The engine's torque, the gearing in both the transmission and the differential, available traction from the tires, aerodynamics, and the sheer amount of the car's mass all have telling roles in performance. (That's the ultra-basic version, of course.)
For most cars, accelerating in a straight line -- which is what horsepower does for you -- is the single least impressive aspect of performance. Throw all those other factors in, and it's absolutely possible for a car with less HP to outperform a car that has more.
Think about the force throwing you into your seat when you hit the accelerator. If you're in one of today's higher-end sports cars or sport sedans -- an M3, 911, or the like -- you can expect to go from zero to 60mph in roughly 3.8 seconds. That translates to an average acceleration of just over 0.7 G's.