I tried to put "Mindful" here. On the idiom lesson, they said that "Aus den Augen, aus dem Sinn" is "Out of sight, out of mind." I figured that "sinn" is mind. Full is "Voll". "Sinn Voll" = Mind-full, which would be mindful... Which I would interpret as being very close to sensible.
Sinn is sense, meaning, signification and by extension mind. I mean maybe that is the original meaning of the word, I don't know the etymology, but as a matter of fact today it is a secondary use. Better remember Sinn = 1) sense -> sensible -> someone with a good mind -> 2) mind.
As DL has it, really in this case seems to me more like a modifier to sensible. As in "very sensible". Your version sounds more like an affirmation that something is indeed sensible. I'm not sure if the original German version carries both meanings like that. I hope someone else can clarify.
Echt means actual in the sense of "honest", "authentic",. or "the real thing". It's stronger and more definitive than wirklich. This sentence would mean more, "you may not have thought so, but it really is useful". Also, I've never really (wirklich, lol) seen echt used as an adverb, it's always an adjective.