exp is not halved at level 20. Subscribers just start getting 20% more exp at level 20.
As a preferred member it stated that my experience went down by half or at least some. I know free to play players get sprint at 15 and speeder piloting at 20 whereas subscribers get sprint at level 1 and piloting at 10.
PSN: Gogetenks "Love seeing you two together again, you always were my favorites" - Stan Lee Spider-Man PS4