Orek does give the majority of the XP for a GR. It was changed this way because groups were running to the RG for the XP and then leaving the game without actually even trying to kill the RG.
However, I think even without the XP from Orek, you get more XP in a GR then it's Torment equivalent.
I believe you get more loot running T13 than GR 60 for sure and probably another 20 GRs higher than that. There are other reasons to run Torment rather than GR; Death Breaths and other Mats, Gobs - singles and those rare Gob packs.
I agree they are likely faster because you don't have to stop to pick stuff up or die trying to pick stuff up. In any case, it feels to me like T13 hurts more than GR 60.