I'm thinking you should glue some pieces of 4" styrofoam to the bottom of your current bags then go for an aggressive ride and see how much gets rubbed off by the road. Then you'll have an idea how much you can go with the extended bags.
I've never done this but I think it'll help you decide....
Assuming that is an ultra with 13" shocks unlike a street glide with 12" and you still scrape with 4" bags?
Mike
Yes it's an Ultra, and yes it will from time to time mostly in and out of driveways with a dip in them, i however have not scraped in leans as im not an aggressive of a rider.. TIP: SKIDPLATES!!!!!