I don't actually criticise the damage boost but the experience bonus you get from causing huge amounts of damage with headshots. I quote user on the mod's Nexus page:

The second major problem with the mod is that it doesn't affect any enemies added with the expansions (first time I noticed that with the cyberdogs from Old World Blues). All DLC packs were released in 2011 and Realistic Headshots was last updated in October 2010.

http://www.nexusmods.com/newvegas/mods/35037/?tab=4&&navtag=http%3A%2F%2Fwww.nexusmods.com%2Fnewvegas%2Fajax%2Fcomments%2F%3Fmod_id%3D35037%26page%3D1%26sort%3DDESC%26pid%3D0%26thread_id%3D250574&pUp=1