With diffuser (printer paper:I'd recommend giving it another shot with a diffuser. I have diffusers on my lights, too, but using the piece of printer paper, as recommended, gave a way different number. I'd really love to see your results.
“Real” par (apex par monitor) 1020
LED white settings: 1270
LED blue/red: > 3000
other test (not maximum light, ramp going up):
“Real” par (apex par monitor) 709
LED white settings: 500
LED blue/red: 1071
T5 turned off during tests