Base your dimensions off of the inside diameter of the hole (not including the size of the plastic wall, this would be outside diameter).
If your inside diameter is 1.5" then assuming just gravity as pressure it will get you about 2100 gph
of flow, according to this chart: GPM/GPH Flow based on PVC Pipe Size, ie, How much water can flow through Sch 40 Pvc Pipe Size 1/2" 3/4" 1" 1.5" 2" 2.5" 3" 4" 6"
That is absolute max. You may be limited to how fast your siphon tubes can get water to that hole (I'm not sure the flow of siphon tubes and how they work in relation to size).
I'm willing to bet the siphon tubes won't move water equally as fast as your drain hole (because then your outside overflow box would overflow).
Then your GPH
can go downhill with twists turns or bends in the drain tube.
If those are the inside diameters you listed I'm going to say it would be around 1800gph.