As stated above, it depends on the custom stunt. However, your stunt sounds like it is covered by the following option in the stunt rules (YS149):
"Get a more powerful effect, but only when you
spend a fate point. Usually only allowed once
per scene.
Example: Killer Blow – Add 3 to the
damage of a Fists attack on a successful hit,
once per scene, for a fate point. This stacks with
any other damage-increasing stunts for Fists."
In this case, the player isn't spending a Fate, but is tagging a free-taggable aspect, which is basically the same, at least to my mind. So the second half of the rule should probably also apply to the stunt in question. Or, at the very least, limit the bonus to once per roll.