IF the fuel is "weathered" - that is it's been at altitude for an hour or more so that the dissolved gasses have boiled out of the fuel - suction feed usually works pretty well and should keep the engines running above 30k.
However, if the fuel isn't weathered the engine driven fuel pumps will cavitate at altitude - engine operation will come erratic - flameout or a surge/stall is likely. Plus the cavitation can cause damage to the fuel pumps - reducing the pumping capability and generating debris into the engine fuel system.
There have been several cases where shortly after reaching cruise altitude, crew members turned off the wing boost pumps instead of the center wing tank pumps, resulting in an all engine power loss. Fortunately, best as I can remember, they were always able to restart the engines.