Performance curves can usually be found in the engine TC reference data or acceptance criteria; I can't say I've seen any for the Merlins but someone doubtless has them in a bin somewhere.
And you're right about torque going up if a lower rpm is set - In fact that's how the prop governor is able to control rpm: it varies the load torque absorbed by the prop.
But shouldn't the old GUMPF check mean the prop is set to high rpm on approach? That does two things: 1) adds a bit of drag by placing the blades in flat pitch, and 2) makes for less to worry about on a touch-&-go or a waveoff.