One thing that doesn't seem to get mentioned enough is that your banner is at the top of your funnel. If you're paying CPC you may not want to drive up CTR just for the sake of it. You'll definitely want to split your demographics pretty narrow, as much as whatever platform you're using will let you. This way you're designing creatives that encourage a conversion after the click. This would hold true even with CPM advertising where the clicks aren't costing you anything.
There are things you can do like providing consistency between the ad copy, design, and colors and the landing page, etc.
Agree on this entirely. Some of my best (highest ROI campaigns) didn't have the high CTRs or other hallmarks of classically optimized campaigns. It's about conversions afterall. The rest are just variables to influence the conversion rate. The point of buying traffic isn't to get it for as cheaply as possible, it's to produce the highest roi possible.