Once the JDM engine is in. The next problem would be getting parts. Aftermarket and Stock. Most Parts people are dumb enough, just imagine trying to get Japanese parts from Nissan. Isn't that an oxymoron or something?
That's not a problem. JDM just means it's sold over there. We have the GA16DE here too so you can get all the same parts at your local Nissan, other than the slight differences that is. say you need a spark plug, a PCV valve, a oil filter... those things are all the same... now if you need a new exhaust valve(s), then you'll have more trouble.
Where would we be able to get a JDM spec GA16 at anyways?? And for how much?? For the cost of the engine and the ECU you could probably buy enough parts to get you those 15 HP so in the end it wouldnt be worth it.
also about the expected power gain, would japanese high octane fuel have a very significant effect?? and if the jdm engine is compatible with the current us spec harness? or if it will, but very doubtful, pass state emissions???