I'm arguing that if you've done productive research for a while and you've got a good LOR, pubs that aren't first author might not help as much as a talk or a poster that is first author at an off-campus conference. If I were an adcom that would be my own opinion, as per the explanation I gave above. Of course, if you can, get posters, talks, and manuscripts, but only having mid-author manuscripts might leave some doubt as to whether you're carrying out a project to something tangible or if your just lucky enough to have a mentor who is. Now of course, the research LOR will still tell the tale even if your reserach products don't.
idk if you're applying in this current 2019 cycle, but it seems like the only people getting accepted even to mid-tier privates now are the EC superstars we muse about. The mid-tiers may have more leeway on stats, but this discussion about research productivity is still helpful info for people with the lower LM's shooting their shot at mid-tiers.
I wanted to get all this out because I spent a while in undergrad worrying more than I needed to about getting on a manuscript in one of my labs.