I've looked into this extensively and it doesn't change the rolling diameter by purely spacing out the rear wheels. How can it? I was looking at a staggered setting for a set of wheels a friend had, and a careful mixture of correct tyre widths/heights will keep rolling radius's the same. By simply spacing the rears out my put stress on things - what I don't know.
The only way you can get the rear wheels to turn more or less than the fronts is by putting different size wheels on the front or rear, that or different size tyres ie, 35's and 40's mixture with same width or different stretches ie, 225's up front and 235's on the rear - even this last option would only slightly change the rolling radius by fractions.