Ah.
The problem here is what is causing the lower power output when the high rpm runners are open at low rpms.
If it's an additional intake restriction that doesn't create turbulent flow, no problem.
But in this case, the loss of power is typically because you're introducing a high volume of turbulent air (turbulent because the runners are too short to smooth it out) at low rpms. Having tried this experiment with my car, I don't see how this could improve efficiency, but it can't hurt too much to try it.
Typically, these systems are operated by simple vacuum-operated throttle plates controlled by a solenoid. Mazda's iteration has the throttle plates open when you disconnect the solenoid. It's easy to find out if Nissan's is the same. disconnect the solenoid and see how it goes. If it idles like garbage, congratulations, the runners are open!
A more effective solution would be to crack it open and remove the throttle plates for the system altogether and smooth out the inside. Or, even better, just switch to a non-MEVI manifold.
With engines like this, it's an interesting exercise to see if the camshafts from lower-end versions will fit... like find a junkyard 2.0 V6 and pull the cams out of that. Should give you the restriction you need, plus the added bonus of better fuel economy all around (at the expense of power... the 2.0 only put out about 165 hp total)
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