If the insulation and the elec. power were both free, you would still not get additional benefits from very thick insulation. The first inch of insulation you put in gives much more benefit than the last inch you put in.
I guess when your attic is adequately insulated you should look for the next largest source of heat loss. I think the attic typically accounts for only 25% of the total loss. Audel's has a pretty good set of books on this subject.