So goes business.
Those fake Greddy manifolds are what we call "Freddy" manifolds.
I've seen some studies on them several years back. One guy cut one in half and showed how horrible the ports were inside. He had to grind away a bunch of material to clean it up. It looked like a porcupine died in the port or something.
The cheap casting was so porous that it was causing leaks in the middle between runners. There was no way to identify this until you really inspected it very closely. Once you apply a ton of boost pressure, it would be blowing air right out those holes... wasting power, and causing the engine to run rich for no apparent reason.
Once he fixed all that and welded it back together, it worked. He then said he'd buy the more expensive one next time and avoid all the trouble.
There are 'good' knock-off parts out there. You have to research them and find people showing good results from them before you try, though. I personally don't run any. There are name brands that are not Japanese that produce great quality parts... as the Japanese gouge their prices really bad.
You wonder why companies like Apexi and Trust are at risk of going out of business? How about the exorbitant prices they charge for their parts? Lot's of the R&D involved is simply buying a part from an American company and stamping their name on it. Then they double the price. I'm not going to start listing specific parts here, but I discovered a lot of dead-on rebadged American parts in Japan when I lived there for 5 years. We ordered as much as we could from the states to save money.
JDM = BS. If you can get the part elsewhere, go for it. Even the Japanese manufacturers are moving all their production out to China. (actually, the likely beat us to the punch, as they are right next door and shipping costs a lot less)