Re: 01 F4I delay/hesitation at low RPM's
I don't have a F4i, but I'm a little familiar with Honda FI.
Do your obvious stuff like you said, I suggest setting the K&N's aside and use OEM air filter, change plugs. My next step would be to check the fuel pump regulator-FPR. Once the diaphragm inside it gets old, they fail, and leak gas from its vacuum line to one of the throttlebodys. Itíll give you all sorts of symptoms, including what you describe.
To check it, lift your tank, locate your FPR, and then pull the vacuum line off. Is there any sign of gas? Donít start-but turn the ignition key to get the pump to prime. Does it drip? It should be bone dry. If it looks like my last one did, then they're about $50 to replace.
I'd probably not use ANY more fuel additives.
Let us know what you find.