I reckon the fan activation load for the SF450 is more like 200W from what I've seen during personal testing.
But, I think the PSU has a separate activation point based on temperature. So as long as you can keep the SF450 cool then the fan will remain off up to around 200W.
If the power supply isn't well ventilated and it gets hot then the temp sensor will activate the fan regardless of load.
I've tested an SF450 with an i5 6500 and GTX 960 which is likely just under 200W load, and under full system stress the fan did not spin up, but this was in a well ventilated case.
I have since tested a different SF450 in the same case, but this time with an i5 6500 and a GTX 1070 (a max load just above 200W), under gaming load the PSU fan did not spin up, I've not yet tested full system stress load.
I've also used the SF450 in about 5 other builds inside the ML08B (same case as the RVZ02), I've found that this case runs quite hot, but I've not known the PSU fan to come on under normal extended usage, although I've not checked in full load conditions, next time I build one I'll check. But I have had one of those 5 systems returned to me with a popped power supply, whether heat played a part I do not know.
