I'm not against the idea but as stand alone it would indeed look tacky... If you could delete the standard third brake light AND replace the side indicaters with the same things (amber light), including deletion of the indicator plastic (so the emblem goes deeper) AND replace all lights with led lights... it just might look really good and modern.
I would go for that, if it were not too expensive to delete the third brake light and if it does not result in an MOT fail...