ur loop will work better if u separate the cpu cooling and gpu cooling and add in another pump.
it will yield better performance. dual loop always better than single loop.
the temp difference when adding extra radiator will be minimal when ur current radiator able to handle ur hardware heat load. Additional radiator wont help much when u already reach the point where the delta temp of ur water is low.
remember, water cooling wont get ur temp lower when the water inside the loop already reaching near or equal to ambient. You will see ur core temp etc still not touching ambient as u must take heat resistance etc from the cpu ihs to thermal paste to water block.
my advice is if u wan go extreme and multiple radiators setup, try built multiple loops each cooling their own components rather than cool everything in a single loop.
anyway nice setup you got there. add in another pump and built another loop bro, maybe 360 cool only cpu and 240 cool the gpu.
.. i'll add in another pump if i got space haha.. fitting all of this is like.. ngam2 edi lol