OK, I'll jump in here.
First, I've never used a mixing elbow like the brown thing. There's a history of them here on the forum getting clogged. Is yours new or is it a used (questionable) unit from eBay?
I have used a riser, and they are fine. Taller is better. The only down side to them is they take a lot of space and require a lot of insulation to protect your boat.
I'd be tempted to try the mixing elbow if it's a new unit. Otherwise I'd build a riser of known performance. Might even think about stainless..
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