DC Fan using less than half of rated current. Looking to overvolt
Stall current probably does not do what you think it does b/c most fans are stall protected.  This means that they limit current when the motor is stalled and won't actually apply the full current.

The upper limit on overvolting depends almost entirely on the components selected and the architecture of the circuit.  If you have 20V MOSFETs, they will probably function reasonably well at 15V (although that is too close for comfort for me).  If you don't know, then I wouldn't go any further.

Depending on the architecture, there may also be a linear voltage regulator involved.  If this is the case, then you would be placing a higher thermal load on the regulator and coming closer to failure here as well.