Is there a way to detect the player name a "/testfor" command has detected?

Solution 1:

What is stopping you from just doing this?:

/tp @a[r=400,score_killCount_min=15] <destination>

You could then easily detect who won, because that person will be exactly at the place where you teleported him/her

Or this in newer versions:

/tp @a[distance=..400,scores={killCount=15..}] <destination>

In newer versions you can then use this command to tell who won:

/execute positioned <destination> run say @a[distance=..1] won