Chris, insulation is not a thing of how much you can put up there its the quality or R factor of what you intend to use.
More is not better, but quality and R factor is.
Something for you to consider, most insulation has a weight per square foot applied to your ceilings. One can apply soo much insulation that you are skating on thin ice with your ceiling coming down.
Instead of applying more insulation, I would consider removing some if not all that you have up there and using a much higher R factor. I of course do not know the R factor that you have up there now, but usually blown in is not as good as batts in R factoring.
What led you to the attic? With 6 inches of insulation up there already, have you checked your windows, doors, thresholds, storm doors, fireplace enclosure, if applicable, and all other areas also?
Not to be a jerk, but how much has your bill gone up from a year ago with comparable temps? Have you had a price increase? This year is much colder than many years before us, so take that into consideration also.