We do not officially support streams at 5Mbps (or 4 Mbps for that matter). Indeed, we have discussed automatically disconnecting streams at such high bitrates when they are detected, and certainly make no guarantee that they will continue to work going into the future. In some ways they are an abuse of what we offer: free bandwidth to bring what you stream to your viewers. That bandwidth has a real cost for us, and until that cost drops further, some bitrates are not sustainable. Compression is a reality for now.
If you feel 1080p has too many artifacts at 3.5 Mbps or less, I would suggest using 720p instead. Anyway, most viewers can't download, or decode 5 Mbps 1080p video - broadcasting at those settings is limiting your possible viewership reach.