Unless you have actually tried a laptop which produces > 42W of heat, it is actually pretty hard to imagine how much noise and heat that actually is. On desktop standards that amount is basically nothing, but on a laptop it is something which I cannot personally accept.
I have the Carrizo reference system, which can be configured for 35W / 42W cTDP. While the performance at 35W cTDP is impressive, everything else is just horrible. The system will run extremely hot, loud and it will drain the battery in no time. Mind you that neither the cooling, size of the housing or the battery are anywhere close to the worst, loudest or smallest you can get with this kind of a system. The system has a dedicated cooling element for the APU, the housing is rather large and the battery has capacity of > 4700mAh (IIRC).
Regardless I use the higher cTDP options just for testing, since anything higher is just too hot, loud and consumes too much power.
Carrizo / Bristol Ridge on 16nm FF+ with better memory controller (>DDR4-2933) on it and a UVD with VP9 support would be pretty perfect