I think that emotional allomancers don't have any special abilities when it comes to empathy, which is why a good one is so hard to come by. Breeze says something to that effect in the first book when he's training Vin, I believe. They have to practice a lot. Just like all of the other mistings with their powers, detection/sensory allomancy excluded.
Mental speed seems to make people see things in slow motion, so it's basically a time-flow alteration as far as your mind is concerned. Your mind thinks that it's going the same speed, hence no mind overload. Physical speed is the external part of that, allowing you to move at the speed your mind is going. You can do them independently, but being really fast without a quick mind seems dangerous, and mental acuity alone will only help you so much. Since metals come in internal/external pairs, there's physical speed, physical strength, mental speed (perceptual time dilation), and mental strength (intelligence). Those four seem to be the most important Feruchemical skills for combat, with weight manipulation being a possible addition to the list.
I could be way off.