Hi…
Unfortunately X-Mirror doesn’t work with proportional edit since the addition of BMesh in 2.63, but you can make it work…
Enable X-Mirror and Topology Mirror… then enable Proportional Edit… now this only works if you edit on the right side of the x-axis…
select the verts to move and adjust as needed… then disable Proportional Edit… press A to select all verts… then press G and Left Click to accept… the mesh on the left side of the x-axis should now match the edit you did on the right side…
To protect mesh in Sculpt mode without masking, just select all the areas needed in Edit mode and press H to hide… then in Sculpt mode they will not be effected… this is easier if your imported object has the vertex groups, then you can just select the vertex groups for say, the eyes, and press H to hide them…
I’m not sure why your getting the stuttering with only 70k polys… my 32bit laptop doesn’t start to stutter in sculpt until around 1.5 mil polys…
Hope this helps… 