You won't lose the onboard (overnight) charger in any case. The power tank and/or Diginow will be an adjunct, not a replacement. As far as size is concerned, keep in mind that the great majority of stations now are limited to a little less than 6kW....of course, that's not going to be true in the future. And as I understand it, Diginow's 9.9kW solution has TWO inputs; if you use only one, you'll get up to 6.6kW, using both will give you the 9.9kW.
I love the Diginow solution, and I recommend it highly. I was one of the first people to install one on my bike, though....well, there's a long story there.
But I don't use it very often. Honestly I've probably only used it a couple dozen times, on maybe half a dozen long(ish) trips now. But my days of wanting to see the entire country on my bike are pretty much past me now.
So I've been thinking that maybe having it (or other high-powered charger) in a case that fits in the top box or a side bag makes more sense, for me. I could then use the power tank for more day-to-day utility, or just remove the extra weight when I don't need it.
I'm on the fence right now. If I was buying a new bike, it would be a tough decision. Might be the best of all worlds -- extra range, faster charging only when you need it.