In forward shading specular highlights appear to be much larger for very smooth materials in contrast to the deferred shader. In 4.14.3 the specular highlights were displaying correctly in forward shading. (See screenshot) Also know that I disabled bloom in the screenshot so you can better see the highlight.
Worked in Binary 4.14.3 CL 3249277
Broken in Binary 4.15.2 CL 3416026
The specular highlight will be much larger and the bloom gets influenced much more as a result.
The visual appearance isn't much different from deferred shading.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-45017 in the post.