What are the Odds?
The odds increase does not seem to be specifically listed anywhere, but I think it is safe to assume for the moment that it is something small, on the order of 5% - similar to the EBI. I'll update if more details become available.
Does it scale?
Based on the DragonVale Wiki, the increase scales with level.
Dragons at level 15 and higher give better odds in breeding "rare" dragons; the odds increase as the level increases.
Which dragons are affected?
Also per a wiki admin in that article, in response to the question of which dragons this boost applies to:
BFS doesn't make that clear. Most Epics are generally rare, but they don't really define exactly what they mean by it.