"The Integral Trees" by Larry Niven is a novel in which people live in a free-floating cloud of atmospheric gas that is gravitationally stable in a multi-star system. They live on enormous trees with canopies on each end, which are blown in opposite directions by the winds (so they look like the integral sign).

That really depends on your quality metric. If you care about computational speed (because there's only a finite amount of time before the heat death of the universe) then the speed of light starts to be a limiting factor, and concentrating all the computation in a small space makes sense.

But pretty great in surface area. Success is all about the metric you choose.

Ha! Relative to what goal, exactly, are planets a waste of mass?

To the goal of sustaining intelligent life. I'm being semi-facetious of course, but scifi authors and futurists have discussed dismantling the planets to build more mass-efficient structures for a long time.