Well 280 mm means more radiator surface and more performace but let us try first with the 240 mm one. I think it is enough for your needs,.
Here is the new radiator and the new water used bought from your website. The radiator was empty, and the bottle was full. I've connected the tubes to the rad, so the system was connected. I'll then opened the fillport shown in the video, and filled the radiator. I've shaked it after filling, to wait for the air to go away. I've did this 5-7 times to be sure.
Test 1) - 10 minute benchmark
Heavy engine benchmark on extreme, with 55% fan speed, the temperature went max to 65. This is a big difference compared to before with 100% fan speed to 71-74c.
Test 2) - 10 minute benchmark
I've shutted down the pc, and right away heard a large 'splash' sound from the radiator. Like some water felt down, or was stuck somehow? I've opened the fillport to check, and the water was at maximum level (almost coming out). I've shaked it a bit as shown in the video, and filled some more. Then i connected it to the machine again, and ran a new test.
This time i hit 74c. I've seen the temperature dropping 6c depending on the scene shortly from 74c to 65c, and back to 72c.
Test 3) Same state as test 2, but then turned fan speed to 100%. Max tempt was 68c
Conclusion. I might think there is air in the system, and the reason test 1 was better than 2, is because the water was cold to begin with. Also, the radiator is really hot.
I've also managed to film the old radiator inside of it after emptying it :
Radiator inside record