Maybe I am missing something, but my B7100 is rated to lift about 1030# and I have an 800# backhoe hanging way out there past the three-point arms all the time. As long as the weight is chained up, and not swinging around, I think you should be fine.
That is an interesting note about tire loading not getting the weight off the front axle. I had not considered that before, and I have loaded tires too. I did notice that having the backhoe on vs off makes a huge difference in steering, even without a load in the bucket. All that weight down low and on the rear axle sure keeps her planted when the loaded bucket is up in the air, too.
As far as 400# vs 600#, think about the distance from the rear axle to the loaded bucket vs the distance from the same axle to the counterweight. You need 2# on the back for every 1# loaded on the front to keep her balanced like she was empty. The front axle is already carrying the "extra" weight of the loader all the time.