In my opinion, they are all pretty much the same, given that the heat is generated inside the case and they are only cooling the bottom of the outside?

The main advantage to me is that they raise the laptop above the working surface and allow good airflow.

I don't rate the USB ones, as they are using power from the machine and that generates heat. It also drains the battery.

Increasing the RAM doesn't generate heat, only using it does.

Your best move that you have suggested is to replace the HDD with SDD.

Using an external drive as opposed to the hotswap would also reduce power demand and hence heat? Otherwise look at SSD, although I haven't seen 1TB yet, and it is expensive.