To answer your question on a simple basis (in a pleasant world away from flow control and high-level sector splits
) the answer is Yes, the leading aircraft should have priority.
ICAO (pretty sure its in 4444) says Aircraft AT the level has priority, not aircraft filing the level. If the 360 man is behind you and already at the level, and in conflict, then of course, no, you won't get it. But the level on a Filed flight plan is about as inaccurate as the filed speed, and we know that; mostly it sits on an RPL (Repetitive Flight Plan) registered a few years ago!