@Owrlly?,
neither the Dell XPS 15 9570 nor the Lenovo ThinkPad P52 do "hardcore throttle", because throttling would be when the CPU can't sustain the base clock-speed value. What the XPS and the ThinkPad can not do is run at the max. turbo clock-speed indefinitely, but that is not throttling.
Regarding the cooling system, the base model of the P52 has a cooling system with two fans and a single heatpipe (higher end models with P2000 and P3200 GPUs have a different cooling system). The Dell XPS 15 has two fans and two heatpipes, so yes, it has a stronger cooling system to compensate for its smaller size and thickness. Despite this, the P52 runs cooler overall.
Undervolting unfortunately is not normally a part of our reviews, it is only done for special interest machines.