I don't think the back pressure difference of a 90 has any real bearing on a system that fills a cast iron tank full of water like the CC muffler system does. For me, I don't think it's an issue at all. I rarely run the motors fast enough where I have any real world consideration about back pressure.
I know when Warren Pateman installed his diesels, he had some concerns about back pressure and they measured it with some sort of an instrument. Thats beyond what I'm doing as a hobbiest. Looking at the system I took out, and the one I installed, I'm not losing any sleep.
In addition, my philosophy of "less is more" is as follows: The fewer fittings and hoses you have in the system, the less likely you are to have a fitting or joint prone to failure. In that regard, less may well provide more
safety and relliability. I evaluated that when I did my installation. If you were able to see the video, you can see a shocking amount of water in the system, and that's how it runs at idle. This is partially due to the flow, and the height of the discharge pipes, etc. At speed, everything blows through that 4-1/2" diameter pipe and if there's any backpressure, it's probably created by the muffler and not by the 90. If you look closely, you'll see that 90 is actually segmented, so it functions a lot smoother than a true 90.
Just a couple of thoughts.