Hey Chris,
I don't know alot about the drain stuff except what we went thru. I don't regret installing it, but want to throw some ideas out there for consideration. I'm not saying that this info is gospel; others can chime in.
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We did a 6" drain, like Bar-O. We do not have any riser pipes, though. It is a very simple,
straight thru the bottom of the dam kinda drain. We did install an inlet filter. Our pond is surrounded by tall timber. I can't imagine not putting a filter on our project. We used a butterfly valve; a very nice pc of equipment. It worked beautifully, but it didn't take much to clog the drain at the butterfly vane right after we installed it. I guess what I am saying is that use of a butterfly valve might very well go hand-in-hand with an inlet filter, particularly if there is any vegetation close by that will litter into (or get blown into) the pond.
Eddie Walker was building his pond about the same time and we were comparing notes on valves. IIRC, he was told that a gate valve (knife action) might collect particulate within the gasket area and hinder closing and/or sealing action when the valve is closed. About this, I dunno. Kinda makes sense.
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At the risk of duplicating your research, I'll link you to one of my self-professed classics:
Drain Valve thread
