Okay, so my measurement device came and I was able to measure the heatsink.
With normal load (desktop, PSX Emulator), the heatsink had about 50°C. With full CPU load (stress test), it went up to about 64°C.
No idea how much the GPU can do, as I don't have a way to stresstest it as well.
Even though this is with heatsink, I don't think heating will be such a huge issue when we apply some small but wide area metal to the SoC.