It's funny - I found my shields worked best in the same rotation position you found. Glad they're working better.Wait! I am behind on updates. I will blame Reefapalooza NY.
I did some tinkering with the shields, rotating them around to different positions around the clockface. Very counterintuitive but we got them to certainly improve things at (face on) about 22:00 hrs. Running 50% now with just a bit of sand moving. We will see. File might be too big to watch.
A while ago, I watched a video of a 22ft tank. I remember they talked about flow challenges as well. It's a longer video from BRS, but might find additional inspiration.