I've been reading a few threads and people stated that mobile pop under provides low quality compared to mobile display so I'm just wondering if it's worth testing out (does low quality = lower CVRs in general?), seems like the CPM's are way higher as well
Low quality in this context doesn't necessarily mean that the frontend CVR is lower, but that the traffic might back out worse for the advertiser than display traffic (less user interaction, more interruption marketing). Even if you are running a SOI offer, at the end of the day your goal is to generate enough paid signups to keep the advertiser happy.
Unfortunately, this has also been my experience so far. Got a couple pretty significant payout cuts when sending 100 % pop traffic, but I know people who make it work quite well while also keeping their quality in check, so definitely worth testing.
Regarding "way higher" CPMs, take into consideration that other than banner ads, every impression is basically a "click" since traffic goes straight to your LP. This brings me to another point worth adding: make sure that you have a bulletproof server/tracking setup before running this kind of traffic. You can get to numbers > 1 mio / day quite fast and don't want your server to die along the way.
"Quality" means different things to different advertisers. Apart from doing the obvious things (remove "free", no adult, no fake alerts ec.) there's not much more you can do.
Many App devs want volume downloads as this improves their organic results. Others want up-sell to their paid apps and services. IMO the majority have unrealistic expectations which is why new advertisers usually come in with high CPI's and then drop them fairly quickly.
CPM costs are irrelevant. Focus on CPI's and see if it backs out at the "high" rates