I plan to set up a CPU + GPU single loop in my Corsair 680X case (i9-9900k + 3090 FTW3 Ultra) and both will be overclocked, although not to extremes.
 
I was pretty much going to go with a 360mm rad at the front (intake), and a 240mm rad at the top (exhaust), which will *just* fit in the 680X case, but it's extremely tight and a small potion of the 360 rad is obstructed at the top (see photos below). The pump/res would likely have to go in the rear chamber..  however I was just having another think about it and I could instead put a 280mm radiator at top and bottom, both exhaust, and keep the 3x120mm intake case fans up the front, which would be better for keeping cooler air flowing through both radiators, and neither would be obstructed at all (again, see photos below).
 
Question is, are 2 x 280mm radiators going to present a significant performance drop compared to a 360mm + 240mm radiator, considering the airflow will be a little cooler with the 2x280mm radiators, and no obstruction?
 
..when I do a basic 5x120x30 vs 4x140x30 it's 18,000 vs 16,800 so really not much difference..
 
I intend to mount the GPU vertically btw.