I wrote a big piece about the team teleporter idea and I'll use a lot of the same ideas here for the team speedzone, because for all intents and purposes, they're pretty similar objects.
Both items serve to (a) add variety, (b) cut down on travel time, and (c) provide an escape for fleeing players. Like a team teleporter, a team speedzone does not impact (a) or (b) much, as it still adds variety and still cuts down on travel time for the player who utilizes it. It does impact (c), by impairing trailing enemies and giving fleeing players an easy way. Like I said in my teleporter post, this would affect gameplay negatively by giving its users an extreme advantage.
However, unlike a team teleporter, a team speedzone cannot* be exploited (at least, in a way not already exploitable by a regular speedzone). Like, imagine if you placed a team speedzone leading into a base. For a speedzone to work, a ship has to travel down a path, and it must be free of obstacles. A team member could use the speedzone to boost, and the enemy couldn't use that boost, but the enemy could still travel the same path and simply fly at a lower speed. So, you'd have to place a laserbeam restricting the path, but then why bother with a team speedzone at all? You could just use a regular speedzone and accomplish the same thing. So basically the "team" aspect of a speedzone is not exploitative, and the only concern is if it gives users an extreme advantage.
The reality is a team member will always be able to use a team speedzone to flee from an enemy and the enemy will not be able to use it to catch up. So to best use a team speedzone, you'd need to try to minimize that extreme advantage. Here are some ideas:
1. Shrink the level. If an enemy can cross the entire level in like 10 seconds, it's not as much of a problem if a player they're chasing uses a team speedzone.
2. Force team members to accept a worse outcome should they use the team speedzone. Have the team speedzone lead into dangerous territory, like an asteroid field or enemy spawn area. So while a team member could still get away from an enemy through a team speedzone, they may be risking their death by doing so.
3. Place the speedzone in such a way that it is rarely used by players to flee, and mostly used by players to travel, like in a base leading out to the middle of the level. Being that it is in their own base, a team member will probably not use the team speedzone to flee from an enemy, but will use it to travel faster.
It is point #3 that really emphasizes the quality of this idea. Consider base security. Turrets and laserbeams can be destroyed and defending teammates can be killed. A team speedzone, however, leading away from the base would provide reinforcements a fast opportunity to catch up to an enemy flag carrier moving at a slower pace. This is a good thing. Fundamental idea of Bitfigter level design is that it should be hard for a player to flee, whether they're weak and are seeking aid or have stolen the flag and are escaping, because it challenges them to survive and defend themselves.
Since a team speedzone cannot* easily be positioned for exploitation, I have no qualms about this item being used inappropriately. Level designers would still need to take care in not giving fleeing players an egregious advantage, but this is definitely doable, so I like the idea.
*Technically, there is a way to use a team speedzone to exploit a path in a way that a regular speedzone cannot. I don't want to reveal it in detail out of fear it will be patched in a future update (although the devs do know about it so who am I kidding). Look at my Year of the Doge level on Pleiades and you can figure it out. I expect this to be a non-issue, as most level designers will not go to that much trouble just to exploit it.