I have a few thoughts on this:
I've always felt that the interior noise of the 405Mi16 was a little high.
and I attribute this to exhaust noise emanating from the tubular exhaust
header. That could be reduced by wrapping with the heat insulation racers
use. I think the mags call this roarty!
As to adding a K&N filter. The drop in element is a safe bet for a tad
more umph. The K&N Ramflow (?) scheme could cause an increase in interior
noise.
The new ROM for the ECU is a good bet.
A low restriction exhaust is a good thing and the increased sound is
behind you. I have a Sebring Suspuf on my car and it's sweet. I drove my
own Mi16 and a 1995cc one the same day and I'd same its got more low end
grunt but mine wails and flies better above 4800 rpm.
Making sure you've not increased tire diameter will be a help, even
getting a 10 mm smaller can help acceleration I'd reckon.
-Jim