what about system agent undervolting?
Most people do not bother adjusting this.
does it help reducing the temps?
Probably not that much or you would see more people undervolting this. If the System Agent does not consume a huge amount of power, reducing the voltage is not going to result in a huge savings in power. When my desktop CPU is loaded, the System Agent only consumes about 1% of what the cores consume. You are more likely to lose stability before you accomplish anything by undervolting the System Agent.
voltage offset is what ThrottleStop currently does I guess?
The Adaptive option in ThrottleStop allows you to request an offset voltage. The Static option is used to request a fixed voltage. On my CPU, only Adaptive / Offset is available for the System Agent. The core and cache and Intel GPU allow both options. I do not have access to a wide variety of hardware so I do not know if the Static / Fixed voltage option works for every CPU or how it works on every laptop. It is possible that Static voltage requests will only work over a certain range like 0.6 V to 1.52 V or the requests entered in ThrottleStop might not work at all on some CPUs. When I am not 100% sure about what CPUs are supported, I try to leave options unlocked so users can experiment. Intel does not share their full documentation about the FIVR with individual developers like myself. This stuff is all a big secret at Intel. They probably cringe every time I figure out something new.
For most laptops, the Adaptive / Offset setting works best. This setting is used to lower the CPU voltage along the entire voltage curve. It does not allow one to perfectly match the voltage to the CPU speed from idle to full speed. Other than that, it works quite well to reduce laptop temps.