Those can just appear sometimes. I have that warning myself. The only time to worry about it is if you have something like MSI Afterburner, AMD "Gaming Evolved", SweetFX, Overwolf, etc installed. If you do, should uninstall them as they can impact performance and capture stability.
Glad to hear the green-dots problem was just the HDMI connection after all! Saves a couple bucks.
Can just run with your current settings and everything should be fine now. If you want to optimize further though, two things. First, disable CFR. Twitch needs CBR, but CFR is just there if you're locally recording and want to edit the videos you've saved with a 'picky' video editing suite, like Sony Vegas. It doesn't make much difference if it's on or off, but it can smooth things out just a little more.
Second, and more quality-based, you have an i7-4790K. It's almost certain that it will be able to handle a lower x264 preset than Veryfast. I'd recommend going down to Faster, then do a test stream at least 20 minutes long of actual heavy gameplay. Watch your CPU temperature, throttling and load. If it's fine, then go down another step. The slower the preset, the more CPU it uses, but the better compression it uses, so your stream's video quality will be higher. You generally want to shoot for 60-80% CPU load as a safe point, with the option to run up to 90% load. Want to leave some safe margin for any game you're playing on the machine (and shouldn't use the Magewell for on-system gaming; Game or Window Captures are much more efficient and lower-overhead), but the slower you go on the preset without over-loading the system, the better. I'd spitball-guess you'll probably be able to hit Medium at least, but do test each step.
Likewise, if possible gaming and streaming at 'native resolution' is going to give you the best possible picture quality on-stream. Consider running your console in 720p mode, cap card the same, and OBS also with a base resolution of 720p. Everything will be much crisper and clearer for your viewers; a downscale is a noticeable quality loss beyond just the lower resolution. It's mostly there for those on-PC gaming on a 1080p monitor who don't want to deal with playing their game at 1280x720; something that really shouldn't be a problem as far as console gaming is concerned, since most are optimized for each resolution level, and many just don't/can't run at actual 1080 with a good framerate anyway (looking at you, XBone).