I have the Liquid Freezer II 280, which I upgraded last month over Corsair H105 (4 years old 240mm rad). The pump is much quieter and the VRM fan is nice, but at the end of the day, the temps didn't drop by a significant number.
Regardless of cooling, AVX tests just overload the hell out of the CPU (90c-100c), and with my motherboard I guess the voltage is not so stable because I get temperature spikes over some cores.
It's very easy to know if you're limited by the cooler or not, if you stress test and the temps spike within minutes, then having higher heat capacitance won't help.
I liquid cool my both my CPU and my GPU with seperate CLCs. My 1080ti has much higher TDP than my 8700k, yet with 140mm rad it rarely goes over 50c, whereas the 8700k has 280mm rad and can go into 80c-90c when stressed. The explanation to this is that heat transfers much more efficiently on the GPU since the die is cooled directly. The air that emerges out of the GPU rad is significantly hotter than the CPU rad.