Biggest Factor For Good SPS Ployp Extension

[Myka]

I would say that provided the basics are being met, flow is the most important, then light. Really, good PE is a culmination of everything.

[Dearth]

This is one of those topics that everybody has an opinion on and it will be hard to get an agreement on as there are a lot of variables from the type of coral, to lights, to water clarity, to water flow and everything in between including fish and inverts which can affect coral extension

For me I feel water flow is important in coral extension as well as whether your coral rely on light or feed off what's in the water column

[Snappy]

I my experience generalizing sps polyp extension without first considering the species and second the vast variety of each species is just chasing a vague & impossible dream. There are lots of sps species that have very little to no PE in the daylight hours and others that are very hairy all the time.
In my opinion the biggest factor is genetics.
Once you have that part of the equation figured out you can then break it down into what enhances PE on a specific coral but keep in mind what will enhance one species of acropora doesn't always have the same affect on another. Consider Red Planet - AKA A.hyacinthus for example. In my experience it almost always has great PE in most aquarium environments good or bad, while other varieties of A.hyacinthus may not have near as much PE in the same tank. As previously stated genetics is the biggest factor.
Also frags of frags from some corals may do better in later generations after becoming "tank hardened" depending on the depth and environment the mother colony was originally harvested from. There are too many varieties of SPS & variables for each for your question to be answered with any real accuracy and can only be addressed in very basic generalities at best.