I don't have a 6,1, but I doubt if the PCIe switch is always the hottest part WHEN UNDER STRESS. Especially if just 65°C.
AFAIK, MFC override the native SMC overheat protection, and alter the fan speed by directly assigning a target fan speed (most other fan software spin up the fan by modifing the minimum fan speed, which keep the overheat protection).
If you make a single fan system base on only one sensor, and pick the wrong one. Some other components can overheat. So, please be careful.
When you have time, you may try to run Prime95 + Luxmark + Furmark at the same time. So that all CPU and GPU should be stressed to limit. Of course, monitor it carefullly when you do that. And make sure the CPU and GPU temperatures are always below the PCIe switch's temperature.
If not, may be you better use some other software to just increase the minimum fan speed a bit, but not make it completely base on one senor.
Of the 5,1, MFC works well because there are multiple fans. On the 6,1, that's another story.