I don't see battery life going down when plugged in on a current MBPr15, even when torturing it w/ recompiling all my ports tree.
Maximum CPU load is not enough - you'd need to run full load on cpu, GPU , hard drive and have the display on full brightness - it really is an edge case.
Gaming. Though that's usually not done at 100% brightness. IIRC my mbp with geforce 750 came with a 90W adapter, but that would also not be enough when running the 45W CPU and the 35-40W GPU at max with ram and screen power usage.