Alex Mercer is strong enough that when punches his fist together he'll make a large crater on the ground, send cars flying and make everyone within a 10 foot radius explode.
As for speed, he is almost 300MPH and as far as reactions, I think he did dodge bullets in one scene. But i'm not sure. Still have the game so i'm gonna boot it up again.
The scariest thing is his ability to walk up in disguise and simply absorb any one and in doing so, he takes all thier powers and memories. However, big powerful beasts in his game that can throw tanks can resist him until they are weakened, so that should mean that any tekken fighter strong enough can resist that for a while.
Again, I have never played tekken, So I can't say for certain but you can't go wrong with alex.