[popcornmix]

Btw.3: In order to watch German DVB-T2, I've started to overclock my RPi3. But it becomes hot and I get some stutter after a few minutes. As I understand, the HEVC decoding mostly (all?) takes place in the CPU, so it would not benefit from overclocking the GPU parts? Would it make sense to only overclock the CPU for this use case?

  Sounds like you are hitting a temperature threshold that reduces maximum frrequency. Might be worth monitoring when this happens with bcmstat.
By default the first temperature threshold is at 60'C. That only affects arm and not gpu so doesn't normally affect hevc, but you can increase that to 70'C with temp_soft_limit=70 in config.txt.

There is another threshold at 80'C where frequency is limited further. If you are hitting this then reducing temperature will help. Make sure Pi has ventilation and isn't on other hot equipment.
Vertically orienting the Pi can help, and some cases (like the flirc/kodi case) are very good at reducing temperature.