It's likely that 1.0GHz version has more CUs enabled so it has 50% score of 1.8GHz version.So I just checked now Apisak Gonzalo scores. He said 1.8GHz revision scored 2x higher score (20K+) then 1.0GHz, this means Oberon, which has same Ariel GPU clocked at 2.0GHz should score round about ~22K on Firestrike.
This would point at something around ~10TF
When we can find out how many CUs are enabled/disabled we would make more precise estimation.