Can I overvolt a motor when the current stays within the limits?
A well designed 12V  BLDC fan ought to be able to run 6~16V. This is the absolute max for the Cap voltage if you do not exceed the current spec then conduction losses and self heating will not increase and motor increases efficiency.


but then bearings wear out faster.


It may fail locked rotor test at this 16V voltage with no air flow.

for ideal cooling, force turbulent flow at max surface velocity over hotspot with vent design, but use duct for laminar flow of heat removal and avoid grid block at blade surface for eddy current noise and flow restriction.