Short answer.. around 440 PAR.. the proof...
Bulbs are 2x Philips 24w T5HO 3000k
Fixture is a CurrentUSA Nova Extreme
Sensor is placed 2" under fixture with black background.
Highest stable PAR reading was 500. It got up to 650 on warm up but then dropped down to 500 before it stabilized after 10 minutes. Average reading I would say is around 440.
Youtube Video moving the sensor around to show average PAR from center to edges.
http://www.youtube.com/watch?v=5azoSZwaa9o