I think that the solar model is a function of the weather file, from memory solar is measured in global horizontal witch is split into direct and diffuse using a modified version of the perez model.
The design day info has model options that are not weather file based. One of witch was developed by ZhangHuang. Huang has done a lot of work on weather data and some initial E+ conversion from doe2.1e.
The solar calculations in Energy Plus solar distribution FullInteriorAndExteriorWithReflections is more useful for calculations of solar heat gain for specific interior surfaces, however it requires convex zones and does not work well with window multipliers.
Solar can be effected by ground and snow reflectivity, as well as transmittance schedules of shading surfaces.
The material properties setting for solar absorbency (inverse of reflectivity) also have effects.
Yes it would be interisting to have solar modeling options, but I m not sure what would be useful in addition to what is currently available.