Ok so the view distance settings in MyLevel and the fog settings are two different thing obviously. What I found on Orel is that I could extend the view distance without a massive performance hit, as I think it is a render based setting. With the fog distance I could reduce the fog down to about 16384 distance (~400m - Orel is around 250000x250000UU or 5kmx5km) and not get much performance increase.
What allowed me the most increase in performance was making sure that ALL of the statics in Orel have the cull distance set. So trees are at 110000UU, bushes around 32000UU small trees 65536 etc. If you leave the cull distance at 0 the engine thinks it need to be drawn even if the fog distance is set quite low, so the static mesh gets fed all the way through the engine and render pipeline until it is overdrawn with fog in the graphics card Zbuffer. If the cull distance is set to something other than 0 it gets removed from the pipeline much ealier in the the process so it really does increase performance.