I don't own any Golden Age comics but if I did buy one I am sure, for the right book, I would be willing to consider anything up to Moderate Professional. For Silver Age, I would only consider Slight Professional (i.e. small amount of color touch) and for Bronze Age up, none at all.
The one thing I would love to see when considering buying a restored comic (though I know in most cases it isn't possible) is a before and after photo. When I read "piece added" it would be nice to know where and how big!