Currents at the base of an elevated, 5/8-wave, vertical ground-plane (GP) transmit antenna are lower than when that vertical conductor is 1/4-wavelength. Does this mean that the performance of the elevated radials is less important in producing optimal radiation from a 5/8-wave, elevated GP antenna?
Elevated radials are required in order to increase/optimise the radiation possible from any such elevated "GP" monopole or whip antenna.
If the elevated radials are symmetrically arranged and lie in the horizontal plane, then their net far-field radiation is zero. However the r-f current at the common point of those radials is not zero — it is the same as the current that flows into the base of the vertical antenna conductor above that common point. All of the useful far-field radiation from such antennas is produced by the single vertical conductor, not by elevated horizontal radials just below it.
Currents at the common-point of the radials for a given, Z-matched r-f power at the feedpoint are smaller for such 1/2-wavelength and 5/8-wavelength monopole/whip antennas. But those currents cannot be zero if the antenna is to radiate anything at all.
Regardless of their absolute value, any percentage reduction in the sum of those radial currents due to the elevated radials being "sub-optimal" reduces the e-m radiation that antenna (alone) will produce, by that same percentage.