The Hub Extenders are expensive Nuru - I'd go for the 4520 instead.
OK this may be my stupid comment for the day, but after reading your post and thinking about it for a moment, I have no idea why JD recommends having the wheels in wide position with a BH installed. When using the BH I have the loader on the ground either flat or turned upside down and basically with the front tires unloaded. On the rear with the stabilizers down the rears are either mostly unloaded or slightly off the ground. Even as such, the BH if hooked on a good stump will drag the tractor backwards sometimes. I could have bicycle tires on the tractor and they would be sufficient.
Maybe I should give it a try, but I doubt that setting the brakes and leaving the tires on the ground will keep the tractor in place andy better. Maybe, I'm missing something important in the way to use the machine, so if experienced BH operators have any advice I'm open. Most of the time I see anyone using the bigger Case etc they always have the wheels off the ground.