I own a 02 F-350 7.3.
GCWR is not usually a legal limit for RV trailers. Your GCWR is 20,000 pounds because of the power, torque, cooling capacities of various cooling systems, and frame strength of your truck. You can hot-rod the engine to get more power and torque, but that won't change the frame strength or the cooling capacity of your engine, tranny, differential, u-joints, etc.
8,500 pounds for the wet and loaded dually leaves 11,500 pounds max trailer weight before you exceed the GCWR of the truck.
With your Banks go-fast mods, you might be able to pass the test to drag more than 11,500 pounds worth of toy hauler with a 2002 dually. Find a long mountain grade of 7 or 8 percent and drag that trailer up that grade at a minimum of 55 MPH. For examaple, I-70 from Denver to the Eisenhower tunnel, or "The Grapevine" grade in SoCal. If you make the top of the pass without breaking something or overheating something and never have speed of less than 55 MPH up the grade, then you might be able to get by with exceeding the 20k GCWR without any ill effects. However, you'll need aftermarket gauges for the coolant temp, motor oil temp, differential lube temp, tranny fluid temp and exhaust gas temp (EGT) to be certain you don't overheat anything.
You mentioned the F-450. In 2002, the F-450 was a lot more truck than the F-350 DRW. Stronger frame, and much shorter legs in the gear ratio are the biggies for GCWR, and why the F-450 has a GCWR of 26k. And tires, wheels, and suspension along with the stronger frame were the big differences in the GVWR. So your mods still don't allow you to be in the same class with a 2002 F-450.
What do "others" do? There are lots of idiots in the world that tow severely overloaded. Don't be one of them. With your tow vehicle, I would put a limit of 12,000 pounds GVWR for any toy hauler I considered. Yes, they make them, but they're not as popular as the monsters with GVWR of 14,000 or more. For example, here's the 2012 Forest River Wildcat 311THX with a GVWR of 11,515: