Gas Questions - Plugs, Airbox, Muffler?

[GK]

Hi everyone.

1. Plugs
I would like some advice on my choices of plugs and airbox filter for my AU 1 on factory fitted gas (dual).

I was thinking of the NGK iridium plugs but at $113 a set I'd want to be sure they would give me a performance gain. Anyone used them on a gas engine? How did they go?

What about the bosch super 4 jobbies? They are much more affordable at about $65 a set. Anyone tried them? Would the NGKs be better?

Also, how long would each be expected to last? I heard talk of 100,000k for the bosch super 4s. Is this right?

2. Airbox filter.
Should I ditch the standard paper job for a K&N or likewise? Will the computer adjust for the extra airflow? Will I get a performance gain on gas? Anyone done this on a gas engine?

3. Muffler
Would replacing the last muffler help? I know I should go from headers back, but i don't have the cash and I've just replaced the whole exhaust with a standard one. I guess I missed a good opportunity on that one!

Any help would be greatly appreciated.

Cheers,
George