Too late--Reliant got another shot off before Kirk could get power diverted to the phasers.
But what would be "too late" about that? There would be no reason to abort diverting power to phasers just because Khan squeezed off another shot. In a long series of shots, too: this last one isn't decisive or anything.
And it's not as if firing at Khan would plausibly stop Khan from firing, not when Khan is shielded and all. Not that firing at a torp would stop Khan from firing, either. But at least it would help protect the Enterprise
when shields obviously couldn't accomplish that.
If Kirk were trying to desperately wallop the Reliant
, I could understand diverting power away from the shields and into the phasers. Channeling everything
into the phasers is a completely plausible tactic even if the Reliant
is somewhat shielded. But if the Reliant
had already just fired a photon torpedo, I can understand the notion that it would be "too late" (and therefore ill-advised) to divert power away from the shields thereby dropping them in order to increase phaser power in a "Hail Mary" phaser shot. Probably better to just keep the shields up at that point instead of diverting their power to the shields .