A named tattoo will result in a 4% higher MTL as of this changelog.[1] Please note that your named tattoo will not start growing when your MTL reaches your tattoo level but rather when your MTL is 4% higher than your tattoo level but as of the aforementioned changelog, you are no longer penalized for having a named tattoo in regards to growth.
http://www.carnageblender.com/wiki/Max+Tattoo