Negligible as in, it only doubles the time taken. It still only increases the amount by about 2 maybe 2.3x. This is still an impossibly short period of time. Human reaction speed is only so much.

And yeah, policing human error isn't easy. However, there's really no perfect way to establish balance in a written competitive environment.

If you totally balance it, it restricts the writing. 
If you try to balance it, it restricts the characters.
If you let it go free, it restricts competition.

Edit: Guns are fine at low tier too! They're fine at every tier because the viability of a weapon that only has a linear aim and narrow strike zone is limited, they're fine at every tier because the viability of a weapon used by a normal person or even a superhuman is limited only to the abilities of the user.