I think that basic turn rate is when you're still. But can't say for sure.
And don't forget that turn rate is affected by power and engines.
Anything affecting turn rate by a % effect on something with such a ridiculously low turn rate is utterly useless and a complete waste of the console slot FAR better spent on several other possible options.
Besides, the better Galaxy-X pilots don't require RCS crutches to get the effect they need from their ship. You only need your nose on your target once every three minutes, which with Evasive Maneuvers and Cloak is ridiculously easy.
Other than that, single cannons/turrets offer a 180 degree arc where you are baring 100% of your firepower on your enemy, and single beams offer a 90 degree arc on each broadside of your ship that 100% of your weapons are bare to your target.
1 Ensign and Lieutenant Tactical slots, you shouldn't be mixing and matching hardly at all, other than to add a Trico up front (all other Torpedoes are useless on this ship).
There isn't a reason in the world to justify an RCS on the Galaxy-X.