@starfisht, it would be a good idea to confirm with your employer/payroll department that they will issue you a corrected W-2 before filing for extension, since a lot of companies due to various reasons may not issue a corrected W-2.
As @MarilynG1 stated, if you don't get the corrected W-2, you can report the amount you over-contributed by entering it in the "Other Income Not Reported on a W-2 or 1099."
Additionally, if you do receive the corrected W-2, then don't report the excess contribution as income since you will be getting the money back. You will only be reporting the earnings on the distribution.
