Answer to "Do these really help?"I had a Fleet-Air filter in my T600 with a 2003 ISX (the good one- pre egr!) I had it on for approximately 5 months (cleaned and oiled regularly) and noticed zero mpg or power improvements. The only change at all was immediate increased silicate levels in my oil samples. Sold the truck with it still installed.
Answer to "Improve fuel and horse power."I installed an air cell on my N14 and it did nothing... actually, that's not quite accurate. It cost me at least .2 mpg. The only thing I see that it possibly does is restrict the flow of air going into the intake.