Re: ok heres another question:
OK, I've done/tried everything you are thinking of.
Aftermarket rear box will be rubbish if it is a "Universal" type. They tend to be straight through fo noise so you don't get any oomph until about 100000000000rpm.
The panel filter is the best bet for all round performance and noise.
I had a K&N 57i kit on the 155 and it was loud, nasty sounding and basically poorly designed. I got better performance with the BMC panel filter.
I now have a Green Cotton induction kit which is a much nicer piece of work. It's actually a little quieter than my panel filter/modified airbox setup it replaced. On the road I don't notice much difference in performance between this kit and the panel filter I had.
Just for the record, I too have a Powerflow and found the low end torque improved. I only removed the corner box and kept the pipe diameter the same as OE. You shouldn't go wider bore due to pulse tuning etc ....blah blah.......