They're not a bad intake, never done a back to back against an Air Gap though. A 2" open spacer would liven it up as well
I actually have done a back to back. Felt no real difference going from the air gap to the stealth, but didn't want to swap back so the stealth has been on ever since.
Only two beefs I do have with the stealth 1) no provision to mount coil like stock and most other aftermarket intakes 2) every single port was a different size, so plan on gasket matching.
Still its a good intake, probably the best readily available dual plane IMHO (Its pretty much identical to the coveted LD4B intake) if you need the heat crossover for cooler climate driving or fast warm up.
