I recommend the ARC / Focal setup.
I went with the J&M Rokkers in the fairing & lowers and the Rokker 500w amp in the fairing for a number of my own reasons that I'm sure you're not interested in (not being a smarta$$, just not trying to jack this thread).
The main reason that I recommended your ARC / Focal system is that I have had some IASCA quality multi-amp systems in some 4 wheel vehicles, where I could run 2 batteries, upgrade alternators, caps, etc... I selected a single amp system because of the stress that multi-amp systems will put on the charging/electrical system of the HD (I ride a lot and like to keep the bike drama to a minimum), and like you, space is a concern for me.
If you can't do it with 500 watts....