Hi, I'm flying a Nebula so I'd like to turn as quickly as possible. I've got Starship Combat Maneuvers maxed on my character, so I was wondering if the +9 I get using the Aegis engines actually affects my turn-speed. The turn speed is at 18 per second regardless, but I'm not sure if the skill applies there, elsewhere, or what. Thanks!

It does not stack for that purpose. The skill itself explicitly states there is a max speed and turn rate you can get from the skill. The only thing the +10 does if you've maxed the skill is boost Tac abilities that are based on it (like Attack pattern alpha, attack pattern omega, tactical initiative, etc).

combat manuvers is on your skill tree...... if you want to turn quickly use an RCS console in your engineering slot.