Ok I play in Hong Kong where we sometimes play Japanese format.
Before the rotation the BDIF was not Gengar Prime, although many played it.
The BDIF was Porygon Link. It re-used Hunter using Porygon-2, 4 Uxie and a ball engine to get a Porygon Z with a bunch of TS-TM and 1 DCE in turn 1. The hunters deplete the enemy bench and when the active is killed, they lose.
I used Machamp Prime/Donphan and managed to get 2nd: lost to Porygon Link. Gengar Prime did not do well at all: didn't even get into top 4.
First Gengar Prime has a bad SP matchup whatever you all say. Some people use Omastar in the deck and of course it fails against SP. Dialga G X would completely wall it with Deafen lock. And Gengar isn't likely to take a prize at all. Even without Dialga G X it is not that difficult to take 6 prizes before you lose 6 Pokemon. Mew Prime is so Flash Impact bait, and Gengar can't do a thing to SP who just Healing Breaths away the damage. Judge or Looker's Investigation, coupled with Power Spray could deny Gengar the necessary Stadium to create the win condition.
Second even against non-SP you can beat Gengar Prime if you know how to. Judge and Looker's Investigation his hand away. Remember there is no Claydol and it is difficult to draw into the Stadium. Most decks only run 2 Lost World and if they run more they become early-game dead draw. Remember you never put down Lost World first because it could be covered by BTS. Trainer lock would hurt the deck badly because: it relies on Hunter/Uxie to draw, so either the deck doesn't get to use Supporters, or it has to use more Trainers, which get locked.
Sure, this deck will be T1. But it is in no way OP, or even as strong as Luxchomp. Luxchomp is so much more versatile...