Those are really good numbers for just having offset bushings! Usually you can't get quite that much + caster with just the offset bushings and stock arms.
It's not so much the rim diameter that makes the difference, it's the width. The wider the tires get the more they tend to track road features like ruts and things. So, adding caster counteracts the tendency of the wide front tires to steer the car with the road imperfections.
As far as the camber goes I've found that up to about -1° of camber there really isn't much change to the wear pattern as long as you're doing some cornering where you drive. Above that and the camber wear starts to show up on the tires. But even then it depends on how much you drive your car. I would venture that most of us "time out" tires before we wear them out because of the amount of miles getting driven. Even the 6 year old KDW2's on my Duster are starting to check, and they have good tread left still. Seems like tires don't last as long as they used to.
With the power steering you shouldn't have any issues with steering effort. I would think +5° to +6° caster would be just fine. More than that starts to effect things other than just steering, as the "jacking" effect of angling the tires so much on turn in can upset the handling.
Ha! Yeah it's not as bad as you might think. At one point I had it up as high as +8° of caster, that was really heavy. But that was actually jacking the front corners and noticeable altering the ride height as I was turning. I cranked it back to about +6°, which was great for the steering effort. But a lot of my daily driving follows "truck routes" so a lot of roads have significant rutting from heavy truck traffic and the rut tracking was noticeable at +6° with the 275's up front. So I went back up to +6.5°, which seems to be some kind of tipping point for the suspension. It tracks a lot less but doesn't jack the car as much, so it's a good balance between stability and steering effort. With narrower tires or smoother roads it wouldn't take as much.
