You know, I'd heard people talk about "feet" of insulation before, but never quite realized they were serious. Then I found a website where they did a radiant-barrier study and some of the homes they worked in had - well - FEET of insulation!
So, I'm going to guess you don't / can't use your attic at all? I tend to pop into mine all the time, mostly when running antenna cabling for my ham radio addiction... Or perhaps you leave a pathway and just bury everything else? I am also guessing the thickness has to taper down quite a bit toward the eaves, so you still have an air gap for air to flow up through the attic?
I've thought about further insulation, but really wondered how much better I'd get. My A/C already barely runs - heck, I have a programmable thermostat and even on the hottest days of summer (100+ degF) my house won't get from roughly 76 when it sets back at 6AM to the 85 setback temp. By the time I get home at 5PM, it'll maybe be 82 at highest. And that's with about 6-8" of Rockwool.
But the attic is an absolute furnace, through which - naturally - my HVAC ducts run, hence my interest in the radiant barriers. Perhaps I'll add more insulation as well, now! I have a very high-pitched roof, maybe I can build a sort of raised walkway to get through and insulate under that...