This is very interesting. As you know I had (still have?) a similar problem except mine favors the starboard side I'm pretty sure. I think you read through my thread from a couple years back, with similar results.

I took most of the same steps to you and I reached the same point. I wouldn't open that passage up that you're talking about (whether or not it made it into my thread, I did try that with little to no effect, if not the reverse effect). I went so far as to grind out the passages in the riser where the hose connects up - in my case it has a series of diagonal channels for the water to pass through.

When I reached this point, the techs at the marina where I was working at the time challenged me with, "What problem EXACTLY are you trying to solve?" In my case, I do a lot of extended idling (while underway) and it was burning up my exhaust hoses by getting up in the 250-300 degree range after a long enough time. THAT was the problem I was trying to solve. So my "solution" of installing VHT silicone hoses that could withstand the temps I was seeing was the most reasonable thing I could think of. It may be a band-aid, but it's the 3M 5200 of band-aids. I haven't looked back.

Like you, I did my homework on the engine side of the cooling system. The only thing I hadn't touched was the parts that are under the floor after the risers and hoses, and I was not going to go down that road. I didn't see another option, and I didn't want to literally replace every single part of the cooling system from the intake to the large hoses, inclusive of all hoses, t-stat, RWP, strainer, manifolds, risers, the works. That might have done it, but what if it didn't?! That's probably $1200 worth of parts all together.

So I'll ask you, what problem EXACTLY are you trying to solve? If the mainfold and hoses are ~200 degrees... so what? Annoying though it may be, if it's not causing damage... I'm not sure I'd keep chasing it.

But boy, if you do I'll be listening.