I've always enjoyed the idea of the theory, and would love it if true, but nothing in the game ever suggests it, so.. no, I guess.

If true, maybe she doesn't go on to become Ultimecia now, since now Rinoa knows and can work not to become her. If Rinoa did become Ultimecia once, perhaps the events in FF8 changed Rinoa's future forever. She probably didn't have Squall in her life when she became Ultimecia but now that she does, R=U = No more.