I took off a Screaming Eagle air cleaner, and put on the Heavy Breather on my 05 Wide Glide, and I could feel a bit of a power increase. It also looks a lot better, and you can hear the injectors when you go fuol throttle! I like it.
Keep in mind if you're adding more air you'll want to add more fuel as well. If you don't already have a fuel management system you'll need it with more air. More air will lean a already lean Harley which can cause you other issues.
Their download is better than no download at all but if it were mine i'd still add a fuel system.
Check with Dobeck Performance 1-877-764-3337 or 406-388-2377, they give members of HDT a discount and their tech dept is very helpful to dial it in to any mods you've done.