I flow tested an unmodified RPM intake manifold using a cylinder head flowing around 250 cfm, and as I recall, average airflow with the intake attached was around 93% of the bare head. The T2 was near the same number.
For reference, a stock manifold will flow around 88 to 88.5% of a bare head at 250 cfm, and modified (enlarged to full gasket size into the plenum), it will be closer to 91%.
Not to sound condescending, but I do agree with others. Simply stating that a manifold flows 300 cfm is relatively meaningless without any indication of measure (lift range, pressure, etc). A more accurate indication of an intake's capability is its effect on peak cylinder head airflow, and that is best represented in a percentage between the two values.
You may want to ask your machinist more about his testing procedure to get a better idea how he arrived at 300 cfm. It is possible that he used a bare cylidner head flowing 325 cfm, connected the RPM, and re-measured it at 300 cfm. That'd be close to the 93 percent value I mentioned early, and is entirely possible!
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