Yes, early benchmarking results indicate a roughly 2x performance increase (compared to installations running PHP 5.3) on the Contacts screen. This is on an installation running locally with the built-in dummy data. Performance benefits will be smaller depending on network latency and database server performance.
Please ensure that your default role does not grant permissions above "Only Assigned" for create, read, update, and delete. Custom roles cannot be effectively more restrictive that the default role, since all users get the default role.
PHP 7 is not supported in X2CRM version 5.4.3. This is due to PHP forwards compatibility issues in both the X2 and Yii codebase. We will, however, be introducing support for PHP 7 in our upcoming 5.4.4 update, which should be available in the next week or two. I've updated the system requirements on our wiki.
While there's a good chance that a small file change would resolve the issue, there's no guarantee. However, assuming the correct file is replaced and a backup of the old file is kept, risks associated with applying a file patch are minimal (you could easily revert to the old file). I would like to reiterate, though, that the simplest solution would probably be to adjust your upload_max_filesize value, assuming you have access to the php.ini file on your server.
Our next update should be available by the middle of next week.