Six.
The extra speed isn't from extra pressure per se, the curve is the kicker, especially with the short barrels.
Here are some traces to illustrate the difference in burn curves.
This is "normal"
This is progressive
At least from testing in the Creedmoor, an extra 100fps will cost you about 10,000 psi with about any powder. So as you can see above you are less than 100 fps slower at @ 10k less pressure.
So is the speed you ask about possible? Certainly possible. Another question to ask, if they would answer you, is testing date and equipment. Many times data doesn't get tested often, and is simply reprinted. Depending whether or not the equipment was modern (strain gauge) they may back down data to be safe. The example is copper crushers. They don't accurately report data above around 45,000 psi. The same reference load shows swings on the order of 20,000. So to be safe, may back there loads down.
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