I wouldn't do that - you will then have an overstay on your record.
If you go with the '7 days to leave' route then there will be no overstay.
You never know what will happen in the future, maybe you want to apply for a Thailand Elite visa or some other visa which requires a clean record - if you have the overstay then you won't have the clean record required.
So I strongly suggest you go and apply for an extension which they won't grant but you will get 7 days to leave.