I think the airflow on these cases is very over-exaggerated. I've got the Matx Evolv and that top panel really puts a damper on exhaust. I don't have a real watercooling loop yet, just an H105. I've got (3) 140mm 2000rpm Noctua industrial fans that fill the remaining mounts. No matter if I have the H105 in the front on supply duty or the 140's, when I take that top panel off, temps drop around 5c-10c depending on load. Probably doesn't help that I have a volcanic electricity gobbler (290x DCU II), but I was surprised to see the difference just by pulling the top panel.
I don't know much of anything about watercooling other than what I've read, but would the cooling improve with a true custom loop with (2) 280mm rads? -- requires some dremel work on the interior of the front to widen out the slot in the PSU tray cover thing, silly that it didn't come able to accept a 280mm rad. It's the same amount of heat, but more focused airflow/lowering the heat density?