What you're doing makes no sense whatsoever. And I'm not sure you really understand what's going on here.
For a start, the z800 does *NOT* use temperature sensor in the CPU, it uses other sensors (including one on the mainboard) to control thermals inside the machine. There are reasons why HP doesn't use the thermal diode in the CPU, but it works just fine.
I'm also not sure why you worry about CPU temperature. 80deg for a XEON X5675 is nothing, and well within safe operating margins.
The problem here is that you pretty much fiddle with parameters you don't understand. The CPU temp in the BIOS shows the temperature *at the sensor*, which is not necessarily the CPU core temperature. The z800 firmware (BIOS) is adapted to interpret these parameters correctly, and to control fans in a way to keep all components inside a safe operating envelope while at the same time minimizing noise. And it does that perfectly fine.
What you're doing is aertificially increasing the fan speed for pretty much no reason. BTW, you could have done the same without Speedfan or any software, as the z800 has a thermal setting in the BIOS that lets you set minimum fan speed manually. Just sayin...
The thing is that this is not a consumer PC, the z800 is a high end workstation that has been designed for excessive workloads. It will run perfectly fine and cool (though not necessarily silent) equipped with two of the fastest processors, 192GB RAM, three graphics cards and four power hungry SAS 15krpm hard drives at 1000% utilization, because it has been designed for that. And it's been proven many times over that the cooling works even under adverse circumstances.
Using some software tools to change fan speed isn't just useless, you also risk component damage as these tools mess up the internal thermal monitoring.
The best you can do is just let the workstation handle thermals itself.