The supplementary battery harness adds wires in parallel with the existing wires, which you can do yourself with a bit of work. I believe there's also an upgraded relay as well.
It does seem to work well for me, or at least it's not something I have to think about anymore.
Honestly, before I installed the harness, I had pretty good luck by switching to an Odyssey PC680 battery which cranked hard hot or cold. But the quality on those seems to have gone down, or at least they don't seem to last as long for me as they used to.
So I had the harness installed by my dealer before a big ride, then remembered that I'd already bought one and just hadn't installed it yet...
If it does turn up I'll post here, as I'm sure somebody can use it.