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Dan, I'm generally opposed to trying to shim joist tops for flattening or leveling of subflooring. Very difficult to eliminate unwanted gaps at the ends of the shims and potential fastening problems.
Masonite would not be my first choice of materials if I did wanna do that, but in an area where it would be perpetually dry it might work OK if properly set in place with good construction adhesive.
My opinion; worth price charged.
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