I can't answer your warranty issues, but CPUs don't normally increase in operating temperature over their life.
The box fan that comes with the FX-8350 will keep the CPU "core temp" at or below 61C under full load. Some aftermarket HSFs are quieter than the OE box fan but they might not have any more thermal capacity - it really depends on the model HSF.
Obviously the highend HSFs do have a lot more cooling capacity than the OE fan, but even the OE fan should prevent excessive temps unless the ambient temp is quite high. The Corsair H100i is only marginally better than a highend HSF but it should be able to keep the CPU below the AMD specified 61C if properly mounted, no air issues, proper fan direction and volume, etc. A closed loop H2O cooler has a lot of potential issues that a quality HSF does not experience.
All of that being said - be sure that you are looking at the "core temp" NOT the CPU temp, which is a thermister mounted below the CPU socket on the mobo. The thermister will read anywhere from 10C-25C higher than the CPU "core temp". Unfortunately most software calls the thermister reading "CPU temp" but it really is not.
Building a reliable PC involves more than just assembling the parts. You need to be able to configure all of the BIOS settings appropriately. This can be quite involved and frustrating as it can require a lot of trial and error with stress testing. It is however often the only means to get a 100% reliable PC.