After plugging that formula into Excel, this is what I get
V Ref Ra Vsig Rb Vin
5 10000 0 10000 -5
5 10000 2.5 10000 0
2.5 10000 -1.25 10000 -5
2.5 10000 1.25 10000 0
10 10000 2.5 10000 -5
10 10000 5 10000 0
So the 5v and the 10v ref voltage give me the same range. I played with the resitor values a bit and it looks like about the best I can get is a 2.5 volt range.