Had a little look see too (although Twitch continues to hate me, and me it! Buffering every few minutes even on VODs, unless I drop the payback resolution to sow unwatchable.)
Overlay looks good all round, nice clean interface. Assumed the empty rectangle on the left is for a chat feed or something similar? My only crit about them would perhaps be that they look more suited to a fantasy/rpg game, but that’s a personal opinion. You may also want to add your stream name and streamer name into your overlay somewhere - if I’m watching fullscreen I have no idea who I’m watching.
Don’t think your mic volume was too low, but was a little confused as to who was speaking when. Without a camera on your face I sort of guessed which voice was yours. Is there a way to show who is talking on screen?
I also checked out your video and display resolutions in the video stats, and as you suggest, the UW output is probably the cause of your black bars? Both of the resolutions look pretty non-standard to me, and I don’t think you’re going to be able to avoid the black bars entirely.
In SLOBS: Settings -> Video: Check the Output Scaled Resolutions and find one that is as close to 720p as possible (unless it’s changed, non-partnered Twitchers are pretty limited in their video outputs and I think 720p is still preferred?) Other than that, I’m not sure there’s anything you can do other than maybe fill the spaces with overlay elements.
While you’re in the SLOBS settings double check that you aren’t rescaling your scaled video. In Output, check the “Rescale Output” option is off.