My CPU Diode and CPU core temps are low after boot (similar to the temps you are seeing in Windows), but they are not relevant to you since I'm still on the old 2.66 GHz Xeon 5150 dual-core chips. My 2.33 GHz Xeon E5345 quad-core chips have not yet arrived. BTW, I chose the E series chips because of their 80 W TDP. Hopefully they idle at a reasonable temp too. The X5355 chips you have are 120 W TDP. The early stepping B3 chips idle around 50 W, but the late stepping G0 chips idle closer to 25 W. I don't know how you can check this in macOS, but you can check the stepping revision in Windows with CPU-Z.
CPU-Z | Softwares | CPUID
CPU-Z for Windows® x86/x64 is a freeware that gathers information on some of the main devices of your system : Processor name and number, codename, process, package, cache levels. Mainboard and chipset. Memory type, size, timings, and module specifications (SPD). Real time...
www.cpuid.com
What are your temps at true idle, long after booting? If relatively low, I wouldn't worry about it.
P.S. I ordered some OEM RAM (with the big heatsinks), and ironically I received some Xeon X5690 chips by mistake. The seller was very anxious to get these high performance MacPro4,1 chips back since they are worth a lot more than the RAM I ordered. So in addition to the OEM RAM ordered, he also gave me a whole bunch more RAM with just the standard thin heatspreaders. They worked fine in the Mac Pro but my RAM idle temps were 15-25C higher
and also caused the fans to ramp up frequently. However, I don't think they made much difference to my CPU temps. I've since taken them out though. Even though I can have 20 GB RAM in my machine with that extra RAM he gave me, I'd rather have just 7 GB OEM RAM with lower temps and quiet fans since this machine is only seeing light usage anyway.
