@Moblyn I too have needed this feature and it's already in the next build.
Instead of .5 pixels, its simply an option titled 'disable mirror on center line'.
This option will disable mirroring the center line of pixels either on the X or Y axis.
So if you had an 8px wide grid mirrored, it would end up 15px wide without the center line duplicated.
Will hopefully get this build release in the next week or so!