@michal89chz
when your system runs a few hours your water temperature will be nearly identical everywhere with a few degrees lower after the radiator. 65°C on your GPU is okay under full load but your water temperature is critical. The only thing to lower it is more radiator size or better air flow. Yur case is not the best one for water cooling because of the front and top which is minimizing the air flow. Since you have removed the top the main problem should be removed. The only things I can see to improve your water temperature is, as my colleague already told you, to increase the radiator size, more water flow. Alternative you can try to run the system with an open side panel. If this will lower your water temperature you can assume that the air flow in your case is still to bad.
when your system runs a few hours your water temperature will be nearly identical everywhere with a few degrees lower after the radiator. 65°C on your GPU is okay under full load but your water temperature is critical. The only thing to lower it is more radiator size or better air flow. Yur case is not the best one for water cooling because of the front and top which is minimizing the air flow. Since you have removed the top the main problem should be removed. The only things I can see to improve your water temperature is, as my colleague already told you, to increase the radiator size, more water flow. Alternative you can try to run the system with an open side panel. If this will lower your water temperature you can assume that the air flow in your case is still to bad.