A few more thoughts after reading the other related thread:

* Have you every watched how the magnetic compass behaves in a turn? Even with a bank angle as shallow as 30 degrees, it's hopeless, especially at low airspeed. Random freeze-ups and reversals in rotation are par for the course. Nonetheless there are specific ways to use it in cloud-- by establishing a southerly heading before entering cloud, or by using another instrument to level the wings before attempting to "capture" the southerly heading via a very shallow turn. Again, these comments are for mid-to-high latitudes in the northern hemisphere.

* Is there really such a thing as an instant-on artificial horizon? Is it instant-on even in a spiral dive? Which models are these? The turn rate indicator I linked to above has an initiation time of less than 5 seconds and is not affected at all by the aircraft's orientation and flight path.

* Re the dirty vs clean question-- I'd vote strongly for "dirty". Anything that creates drag tends to dampen the pitch phugoid. Sudden changes in bank angle-- due to turbulence or wrong pilot control inputs-- pump energy into the pitch phugoid. This is a huge problem when flying "partial panel" in an aircraft with relaxed pitch stability. Drag is your friend even if the G-load at which the airframe breaks is reduced by several G's. If you are pulling more than 3 G's or so in cloud, you are unlikely to be getting the aircraft back under control anyway-- especially if it is clean.

S