Smaller machines tend to draw at a faster rate- so you could blow through more mix if you're not careful.
In general, it should take about the same amount of solution to properly clean a house, regardless of the machine you're using. The exact amount will vary depending on level of algae/mildew growth and the substrate (type of siding/surface being cleaned).
With the higher draw rate of the 5.5, soaping really shouldn't take much longer than if you were using an 8gpm.
Where a larger machine really pays you back is when it's time to rinse.