Re: Beating to windward
11 degrees sounds reasonable to me.. not all boats can achieve that with interference from shrouds, track/car locations, etc. Others can go closer to center than that.. but may not benefit fully due to the various factors already discussed.
As far as the main is concerned, often the optimum angle is determined by its proximity to the headsail otherwise excessive backwinding can result - although on many boats some backwinding isn't necessarily 'slow'. We generally choose our 'boom angle' based on the wind strength and heeling angle. This is where a good vang, and an easily adjusted mainsheet and traveler come into play.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)