The current implementation of the "sun disc" is quite simplistic and as you have noticed it does not reflect correctly or antialias properly, among other issues. Unfortunately I'm not sure that these things can be fixed before release as we are essentially in "feature lock" at this point, though they will certainly be addressed in the future. For the time being you can work around it as others have said, with reflection spread (or specular roughness if using a reflective shader), or by putting in a luminous sphere for your visible sun.
- Oshyan
