thank you guys! :)
yes the additional radiator help to control the water temp. under full load.
Meanwhile i've made some minor modification

:
- switched from the RX240 to te RX480 on the top of the case
- added some Koolance VL3N quick disconnects on the external radiators, they are now full modular
- replaced the fittings under the GPUs and the ball valve with a Bitspower one.
- now i'm using Mayhems Pastel fluid.
i've also replaced my old power supply (noisy under load, tagan bz900) with a Seasonic 1000 Platinum
some photos