<script defer src="https://use.fontawesome.com/releases/v5.0.0/js/all.js"></script>
<script defer src="https://use.fontawesome.com/releases/v5.0.0/js/v4-shims.js"></script>
When you do this some changes that take place are for example from what FA says the icons inherit text size and that is shown when adding the lines you added. So for example your usernav icons become small because they inherit the text size and also for example your check boxes icons like the checkmark itself gets very large. Obviously IPB will need to make adjustments to all these areas when moving into FA5.
So yes, the code above works, but some areas will appear different from the original IPB design intention.
I personally love FA and am positive IPB will upgrade us, question is when and I think there's more than enough time to add these to 4.3 possibly.. I say possibly here