I think you're understating the maneuverability a little bit.Not at all. The phantom has a TIE fighter dial with a tridirection barrel roll 2. What makes it unpredictable is that you don't know which maneuver it's selected. But that maneuver is locked in in the blind planning phase. Likewise, the phantom doesn't know the maneuvers of your ships. A phantom player has a choice of three destinations when he decloaks. Usually there's only one he actually wants to do. In that sense, the phantom has much less reactive power than the TIE interceptor.
They both are of course subject to being locked in during the planning phase, since there are typically only a few maneuvers that are logical for any ship during a game. The key difference in the Phantom and interceptor dials is the 5 ahead (which the phantom can mimick), the 2 green turn, and the 4vs5k (which the phantom has open to it as well).
The fact that it can potentially start a maneuver from 3 different positions (4 if you decide to not decloak) I think makes it more manueverable overall, without even adding the possibility of adding a navigator to adjust further.