If theres rust coming through on top be prepared for lots of rust underneath. Rather than get it repaired I'd put the money into a carbon roof panel.
as muz says really. when rust shown around the sunroof on erst's its usually because the pannel has rusted through from undernieth. need to take the headlining and sunroof out to see how much damage is done.
from what i've read on here the easiest solution is to cut the pannel out and fit a carbon roof. if you don't like the look then get it painted.