Forward shading will most certainly have a negative effect on the hair. The hair shader is making heavy use of the Dither AA. For this to work correctly you need to have Temporal AA enabled (which I assume you don’t have, correct?).
Also I am unsure how translucency sorting is done when using Forward Shading. This could be a potential issue, too.
So if you really want to use Forward Shading I would actually suggest creating a new hair shader from ground up to deal with those issues.