Servos, firewall, and simulator should not really affect your planes flight, unless your servos are terribly under sized.
Expo may be a factor depending on how reactive your plane is. If it reacts a lot with little input you need to add more expo. For 3D I use a lot of expo on my rudder.

Can you give some details of how your plane flies or fails to fly? Or what problems you are having in flight? Also, what plane are you flying? What simulator are you using and what plane on the sim are you flying?

It looks like you have own a few 3D planes, is there a maneuver you are struggling with? Hovering perhaps?