Does upgrading RAM significantly increase power consumption or heat?

Not noticeably. The difference between 8GB and 16GB of RAM is maybe 1–3 watts at most — less than a nightlight. RAM is one of the least power-hungry components; doubling capacity adds only 1–2 watts. The real power hogs are the CPU and GPU. If you need 16GB, the performance benefit far outweighs the trivial power cost.