The bonus for singing in your preferred repertoire is worth serious consideration because it applies to every song you sing, but that being said, it's a very small bonus.

Ergo, if (for example) you're not having much luck with grand nocturne even if you're in a tragic repertoire, that's pretty much what your grand nocturne would look like even if you switched your preferred repertoire to tragic and sang it in tragic. Your problem might be something else entirely -- low charisma, crappy instrument, etc.

Generally speaking, running comedic you should expect most of your support/healing songs to be good and everything else, not so much. Killing Joke brings up the offensive stuff somewhat but you'd still be more effective singing, for example, apoc in epic.