That's wrong. Fglrx _is_ faster because it's much MUCH more sophisticated than open source drivers. Oh, and fglrx also supports OpenGL 4.x, while Mesa is still stuck at OpenGL 3.0 (it might get to OpenGL 3.3 by the end of the year).
Phoronix regularly compares fglrx against the open source drivers. Fglrx almost always wins on rather generic benchmarks.