Side note; found this in the
GEnx TCDS:
FUEL PRESSURE LIMITS AT THE ENGINE PUMP INLET
The limit is from minimum fuel pressures of not less than true vapor pressure plus 5.0 psi
to a maximum of 70 psig.
For the GEnx-1B, on the 787 aircraft, the minimum fuel pressure limit is extended down
to minimum fuel pressure of 3.5 psia and maximum vapor-to-liquid ratio (v/l) of 0.45 for
up to 60 minutes followed by up to 600 minutes with minimum fuel pressure of 3.5 psia
and a maximum vapor-to-liquid ratio (v/l) of 0.28.
3.5psi absolute seems like a fairly solid vacuum when dealing with liquids, and nearly half vapour. I see no conditions saying a thrust reduction is acceptable, or that these don't apply at takeoff.
It seems that the 3.5psia minimum applies to both 787 engines, whereas other engine families and the same engine on other aircraft only need to operate to 5psia.