Addressing the initial question, "Can coasting be just fine?" I'd say absolutely -- I'm sure that the potential exists for a student to coast through all of K-12 and then magically readjust everything necessary to succeed in college. I just don't think it is worth the risk to wait that long to find out the answer.
+1
I had a similar experience, and although my parents did their best within the framework they had (I got 2 grade skips by 6th grade) I never actually worked until after high school. By the time I had reached 6th grade I was fine bringing home B+ and not being the best student in my class -- as long as I could keep reading under my desk without anybody bugging me.
The competitive engineering prep program I landed in out of high school was a rude awakening, but by the time I was there there was no time to 1) learn how to work and 2) actually do the work. Some of my bad childhood habits are still having an impact on my work, twenty years later.
Of course I was very resistant to any pressure put on me to perform from early on (refused to learn how to read or count past 20 until I started "real" school...), so not sure things would have been much better if my parents had tried to correct the coasting
