How much is too much without sacrificing MPG and/or wear and tear on the suspension or tires?
You left out one part of the equation -
how much are you willing to sacrifice?
ANY amount will impact the above mentioned attributes. If you add an amount small enough to NOT sacrifice any of those attributes it's extremely unlikely to add enough benefit to be measurable, let alone noticeable.
That said, one of our employees has a fairly long, steep and not-very-smooth driveway, that he plows with his 2500. He has a concrete block that he puts in the back (behind the axle for maximum effect) every winter. Not sure on the weight but it's probably 600-700#. He claims it makes a big difference, but he also tends to exaggerate.