I did indeed run the HARC ram air kit.
And it's also true that setting became a little tricky after that, but it did make a difference on fast circuits.
At my local (Bira International) circuit it increased top speed from 130mph to 133mph. Longer circuits such as Sentul in Indo would gain as much as 5+mph over a non airbox setup.
Sealing is of course the most obvious, but most overlooked thing on the ram air. The sponge that sealed the box to the tank needs changing regulary, as do other grommets etc to be sure that it's fully pressurized.
Setting up would mean a couple of MJ sizes up on the non box, and as Andy says a float height change of 0.5mm to compensate for the pressurized fuel was also needed.
Not worth a bean in performance for the average, go to the shops road rider, but if you need that extra mojo in the race then it's a must have.
Worth lots of points on the poser charts, so not so much as a must have, more of a necessity.
Will we see ram air on the NSR300 to put it firmly back to the top of the charts where it belongs?