Id heard over 10yrs ago how bad they are stock. 3yrs ago I saw mine and my brain exploded. "Not the Godzilla" i said. But these days i realize a restriction at that point is more of a top end restriction, then a response killer (think WRC restrictors). I still believe Nissan furiousally developed the GTR with Group A solely in mind; sketchy manifolds would limit power for the Gentleman's agreement, but allow race teams to port match IMO, any whoo...
I'd truly thought mine were a dud batch. Researching found it was v.old news to the japs. Mine looked like Jimx's. Manifolds went to Smokey; I went single.
Hrrrm, the whole "stock vs aftermarket stainless"... Iv seen too much info from Japland, US and here that says when the stockers are abrasive ported (extrude hone, power port woteva) they ~match aftermarket ones for power & response. Being cast i would think they are more durable / retain heat. If i was doing twins, it'd be a no brainer in my head, but meh, im not getting into that one.