I've been rendering some clouds for several projects related to Vue itself, and figured I should post some here to show the improvements in Spectral3.
By far the best things I've seen are control over the free floating cloudlets created by the roughness and detail settings, and the major, major, major improvements in light penetration and diffraction.
Probably the same as 7.0 to 7.5 upgrade, I think. 8.0 is a complete new version, so you will get upgrade prices, but only versions up 0.4, like Vue 7.4 are free upgrades. You pay for the 0.5 or 1.0 upgrades like 7.5 or 8.0. That's why when you purchase a maintenance plan, you get two versions (for example, if you buy it for 7, you get 7.5 and 8.0) for free.