I asked Paul Howe about this, and he's reserving comment until additional information comes in. At that point, I suspect he'll pen an article on the topic.
His initial comments were basically: suppress if you can, and maneuver. If the ceiling is concrete, you can bounce rounds off it to get to the rearward parts of the room. That was all he mentioned in our brief e-mail exchange.
The minuscule amount I know about high-angle shooting says it will cause you to hit high, due to the fact gravity only acts on the round over the horizontal distance, rather than the actual (I think "hypotenutical" is a word?) distance. [Source] So if you haven't trained in high-angle beforehand, that sounds like a problem.