I've had an interesting experience. Right after I got my bike ('97 1100GS) I bought a Leo Vince exhaust and have ridden that way since. I also did a jumper in place of the CCP, 30-87a.
When I got my IICE Air, I didn't notice a huge improvement. A bit smoother off idle and low throttle, but that's it. I also noticed my mileage was shite.
Yesterday I put the stock exhaust back on to see if it would help the mileage. I have no clue whether it will help the mileage, but DAY-UM!! The bike is now soooo much smoother and has a LOT more low end power.
I was really surprised.
I set the IICE back to the "zero" setting and it felt fine, but lost a bit of pep. Put it back to -20 and it's got ZIP again.
I was really surprised by the improvement going back to the stock pipe made, but even more impressed by the amount of difference the IICE makes with the stock pipe.