I have a 2.5" Redback exhaust system installed, and also looking at having extractors put in as well. If I were to install a 12mm manifold spacer, and a throttle body to go with it, will I lose power, or gain power?

Given the modifications you're planning on, I wouldn't worry about the extractors. I would recommend a 12mm (preferably a 25mm) manifold insulator first, then the plenum spacer. This aside, you will gain power regardless. With the larger though, if you have an auto, you may need a tune to suit depending on how you drive. This depends on how you drive (cruising/light throttle conditions) as the shift point characteristics would have changed .In saying this though, we can supply a tune to suit.