My lights are actinics and my tank is 16''
firstly, actinics do not help with coral growth. They're for color for the most part. So if that is your only light, you wont grow much of anything. Might stay alive for a while, but nothing will grow.
As for the height... totally depends on the type of light you're using. I'm assuming its not a metal halide since you say its actinic...they dont make an actinic halide.
I had 4x96w PC
over my 40 breeder (which was also 16" high). Growth was "ok"...but not great. Color wasnt great either and i was somewhat limited to softies and some lps
just browned out but stayed alive. I switched to 4x39w T5 HO
and that was a completely different story. I grew everything i wanted with AWESOME color and AWESOME growth rate. A thousand times better than pc
Sorry for the rant...basically what i'm trying to say is...there are too many variables to tell you exactly how fast GSP will grow...light being the most important. The brighter the light, the better for GSP. I've had gsp go from a half inch piece to a foot across in less than a year under good lighting.