You mentioned that you have modified the size to 400MB, but there may be other parameters that also need to be configured,particularly with PHP memory limits. You may need to set those considerably higher. I have attached a link below that explains how to upload files larger than 2MB. In the link are details of what parameters will need to be adjusted for PHP as well as inside your IIS environment.:
How do I upload files bigger than 2MB / How can I increase the upload file size limit?
If you should find that you are still having issues importing large files, I would recommend using the fault wizard within the CMS and sending us the logs.