I'm not saying Gengar was
the play (In Masters, at least), but I don't think you're giving it enough credit. There was actually 2 in T8. And I must be missing some sort of secret tech in Luxrape because, as far as I know, it actually doesn't have that great of a match-up with Gengar.. I played against a Luxray variant myself, whiffed energy the first 3 turns, and still won with my opponent having 2 prizes left. I'm not the only player actually, so don't just take my word for it, there's two factual Nats reports, one of them being a T8 Gengar player:
http://pokegym.net/forums/showthread.php?t=104193
Notice he plays FOUR DPL's in Swiss, which I'd say counts as a Luxray variant, and also a Luxrape in T16.
Another...
http://pokegym.net/forums/showthread.php?t=103935
He beats FOUR Luxrapes in Swiss at Nats, and another in a side event.
These were the only two Gengar reports I read, but I'm sure that's pretty concrete evidence that Gengar has a decent time with Luxray and friends.
...
Seniors ... ended up having 1st, 2nd, and 3rd all as Gengar variants. Notice 4th as well, a Luxrape that lost to Gengar!