Light shelves are really starting to hit the limits of E+ because the best E+ solar distribution calculation option is really only able to model a single light bounce on the exterior. It doesn’t model any reflections on the interior, although E+ does decide how to distribute the interior solar between the surfaces of the zone using the angle at which the exterior light enters. So, when modeling an exterior light shelf, you will be able to account for the exterior light bouncing off the top of the light shelf into the window above the light shelf. However, there will be no accounting for the effect of light bouncing off the ceiling of the zone to the other interior surfaces of the zone. To account for an effect like this, you really need to run an annual daylight simulation with Radiance as you have mentioned.
More info on the assumptions of the E+ solar distribution calculation can be found here in the E+ input output reference: