The overall behavior of Millepora Colony to Colony can differ quite a bit. I have super shaggy ones and others with what looks like nothing for polyp extension until you magnify them and see they're there, but smaller. I wouldn't sweat it.
I have two colonies with that exact growth form pictured to boot and they are nice. Don't sweat that either. They are going to do what they do and that's that.
300 par should be just fine, but they will take quite a bit more. But even that is iffy because I have colonies near the surface and also on the very bottom and there seems to be no problem either way. Millepora (or at least what is being classified as that) is usually very adaptable and resilient. They will completely change themselves to gather the frequencies and amounts of light they need.
The one on the very bottom is a rainbow millepora that is currently acclimating.
Look at the red one...the polyps look like they're barely there...magnify them and they're just fine.
These are all millepora of mine. The best thing you can do for them is to place them and let them do their thing.