LSI keywords are only valuable if Google views them is being industry/niche relevant. If you want real ideas go Google the keyword you want to rank for and see what those on the 1st page are using in their on-site writing. Just because some word is a synonym does not mean it is a valuable LSI. Think of an LSI as "industry terms." So if you are ranking for the keyword "football fan site" you'll use works like punk, QB, touch down, and things like that. Related words to your niche... not random synonyms. Essentially you want your anchor text to be niche related.