The "6100lb" rear axle on my F250 has all the same hardware that makes a 7k axle on an F350 except 2" blocks and overload springs. The blocks don't increase suspension travel; they just jack the truck up in the air. I put air bags on that serve the same purpose as the overloads but ride better when you don't need them. The 20" wheels are the same ones on a 7k axle and the tires are rated for 7500lb combined. You can put an 8lb label on a 12 lb sledge but it's still 12 lb.
Just because someone puts 7k on a 6k axle doesn't mean the wheels are going to fall off.
edit: I made the assumption he would increase pressure to cover the added load but now I see that's what you were worried about. And, I'm not advocating overloading... I will never be near my 6100lb rating but know I have more to spare.