According to LOAF's gun chart, the unboosted Steltek gun does 10.0 cm of damage per shot (compared to 10.0 cm for the Tachyon cannon), while the boosted Steltek gun does 19.0 cm of damage per shot, and has slightly faster refire.
As I understand it, the Electronic Counter-Measure units have a fixed chance (25, 50, or 75% depending on the model) of spoofing an enemy missile's target lock the moment that it is fired. If the spoof attempt fails, it will not have another chance to spoof the same missile, so all you can do is hit the afterburners and try to dodge it (easy in a Centurion, but very hard in a Tarsus).
Privateer turrets, unfortunately, do not autofire. Within the WC universe, it is implied that Confed turrets have human gunners (many mentions of turret gunners are made in WC2). However, in Privateer, you are flying solo, which means that there is nobody else to man the turret. It does suck, though, that your turrets don't autofire--it takes away half of the benefit of having guns on the rear of your ship if you have to abandon steering your ship in order to use them.