Engine power determines potential top speed, though it isn't a direct correlation as doubling engine power doesn't double top speed. Your slider controls what percentage of your top speed you're currently going, and you have full maneuverability at any speed above 25%. Your defense bonus comes from your current speed, so a high potential but holding still results in no bonus (a negative bonus for holding still, actually). If you want to rely on speed to protect you, gotta keep moving.
Full reverse speed moves you at 25% thrust, so same defense bonus as going 25% foreward. However there is a power drain that slowly builds magnitude the longer you stay in reverse, so don't stay moving backwards for long without taking a moment every couple of seconds to throttle up to zero again and clear the drain, then resume backing off. Helps to learn to use the QER (or keybind your mousewheel) for throttle control and ignore the slider as anything other than a gauge.
There's some discussion about there being a sweet spot for damage at about 4-5kms from a target. Moving backwards would probably move you out of the sweet spot.
For Escorts using cannons there is. I forget the math but you dps drops dramatically at ranges greater than 5km. So the best thing to do is make strafing runs. Charge right at your target, open fire at 5km and then bank hard and come around again.
There is also a sweet spot on the engine slider for maneuverability. In my defiant it's just under 50%. So what I do is head at a target at full throttle and then drop down to 50% before I bank.
Engine Subsystem power determines your potential maximum speed. The other factors are the Mk of the Engine any [Spd] attributes, Captain skills and skill modifiers. And there is a limit of +50% defense that you can get from speed alone. You'll the speed defense cap at around Impulse factor 24 (as measured by the throttle pop info).
Also note if you are not moving, your defense from speed is -15%.