It helps, how much really depends on who is evaluating you.
The number 1 thing T20s give you in my opinion is the ease of access to opportunities and connections. It is not that these aren't available elsewhere, but if you go to a "really good" med school, it's easier to get email responses, likely have good and abundant in-house research, the faculty may be well known and can write LORs or make a phone call for you, etc. Again, the nice part is this is possible for pretty much any med student, but you may have to work harder than people at T20s to get it.
I don't think anyone would take coming from a T20 school over a first author pub or LOR from a nationally renowned physician. But it does help, however big or small to come from a "better"/higher ranking school.