No surprise there. Raptor's towing is limited by its suspension, and they aren't going to screw with that just to bump up the towing capacity. Maybe some fine tuning will add a bit, but they aren't going to stiffen up the rear.
My guess is that towing will increase some just because of the lighter weight, but not the full 500lbs.
I think if you're going to do much towing, you'll still want to check out aftermarket rear springs and/or air bags. In my book, 8000lbs max capacity means the comfort level is around 5500-6000lbs. You don't want to tow much at the max nor much over the weight of the truck.