I've seen no limit on AI transport range at all. This has led to some fairly amusing moments, including watching a bulrathi transport travel the entire length of my empire on a huge map to get to the last of the hated darlok's worlds on the other side of the map. They only managed to scout the darloks world when they were allied to the extinct meklar some 100 years previously and had no range better than r4 at that point and yet they somehow loaded up the can of bears and sent it on a 20+ parsec trip.
Just assuming that any world the AI scouts is going to get troopers until they lose some in orbit isn't a bad way to play.
EDIT:typos
Just assuming that any world the AI scouts is going to get troopers until they lose some in orbit isn't a bad way to play.
EDIT:typos