10psi
"Hits a wall at 10psi"
Since you've eliminated all the other variables, I'd check the exhaust pressure going into your hot sides. If you're seeing over 20 PSI there (2:1), then you've found a restriction. The closer to a 1:1 ratio, the better. As a side note- do you have any EGT probes? I'm curious as to what your gas temps are hitting. Do you notice a more-than-normal rise in IAT when you're on the boost for a while?
I agree that your .81 housings are about right & should work fine, but you might be right at their limits already. I think that motor is trying to move alot more air than it currently can, hence the 'wall' effect. I think you can get 1.01 housings for those; that might be worth a try.
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