There are some issues. For starters, the Jolteon with 391 speed will be always outsped by pokemon within it's same base speed tier (130), and who are running a positive speed nature, and with max speed.
For example, a Jolteon with 394 Speed will always outspeed a Jolteon with 391 speed. So, that means that in a contest on which each Jolteon Shadow Balls each other, assuming their SpA are the same, there are no crits, or no SpD drop, then the faster one will always KO the slower one first.
However, not much non-scarfed pokemon have a 130 speed base tier, so, it's not too big of an issue.