gman, i have 58" bars on my 04 suby outback. they do overhang a little bit, but it is ok for me. my wife bumped her head a few times when the rack was newly installed, but it just boils down to a training issue. once you are used to avoiding the bars, you don't hit your head. and in all seriousness, they do not overhang that far.
as for carrying capacity, i have a rocket box on my 58" bars, and can carry three boats stacked with my yakima stacker. this is the single bar stacker with the hooks on the end, not a loop style stacker like thule. without the rocket box, i could easily get 2 more boats on, but i don't think 6 would fit. I think 4 boats would fit on 48" bars with a single bar yakima stacker (might be tight with a loop stacker), and it would certainly work to put them flat (side by side, 2 high) without a stacker.
the biggest pitfall of putting yakima on the suby roof rails is the rail rider towers. i think these towers suck. as i said earlier, the racks have been on the car less than 2 years, and they have shifted position and needed to be repositioned/retightened twice already (and they need it again now). i can't figure out why the towers slip on the rails, but they just do. i am not concerned with them flying off, but they just shift a few inches over time. later.