Do the speeds selected on this page influence just the "planning for the FMC Vnav operation" or are they hard limits that VNAV will never exceed?
They limit FMC commanded speeds (the bug). They do not limit the aircraft speed in VNAV PTH.
Assuming an ECON DES with a high CI, the FMC commanded speed will be .79/310.
If you are above the path, it will revert to VNAV SPD with bug at .79/310. The aircraft will fly this speed.
If you are on the path, it will give you VNAV PTH with bug at .79/310. The aircraft will follow the path. The speed may exceed .79/310 if required to stay on the path. Drag or another mode may be required if speed approaches VMO/MMO.