In SS08, so far as I’ve seen in missions I’ve built, AI ships travel at their maximum speeds.
Overtaking is generally not a problem because, if you make the path wide enough, you can offset the ships so that they may pass each other.
You can’t adjust the speeds of AI ships in the mission, but you can run them faster in the Editor to check that they overtake without conflict and make it around corners.
I doubt that we will see individual control over AI speeds anytime soon. There are more important features, such as making EX waypoints optional for mission completion, and the ability to fix damage to engine or rudder.
Marty