The siphon looks great, but I think I'm missing something. I don't quite understand how your 10" siphon is giving you 880 GPM more than your 15" standpipe or drain. Does the 10" siphon have significantly more head?
This is all from the top of my head, as I haven't had to do this kind of work for a long time. But, every foot of vertical drop will only give you about 0.45 psi per foot, which is a major contributor to how much volume per second you can expect. With just gravity and a moderate drop of 10-20 feet, I would guess you are getting somewhere around a total of 1000-1200 GPM total with a 10" schedule 40 pipe -- unless you have something more in the system we aren't seeing. A 15 inch pipe, with the same head should be giving you somewhere around 2500-3000 GPM.
I think we need a little more info before we can give you an answer. Lastly, the bottom of your emergency spillway should probably be no less than level with your standpipe and sipon -- and probably several inches higher. If the emergency spillway is over-growing with plant material, think about adding several lengths of 8-16 inch corrugated drain pipe to it.
