Arctic MX-5 thermal paste will decrease the cards temperature by a whole lot.
Get some Vodka, a few Q-tip swabs and clean debris off the fans.
Whats your GPU fan set to? Maybe it's set to auto. Is the rig near the window? If it's in desktop case, try removing a side panel.
Also if it's plugged directly into the motherboard it tends to be hotter then GPUs using risers.
Change the thermal paste can reduce temps from GPU, but not from memory
In some cases they can even increase tempsfrom memory because the person need to disassembly and assembly the thermal pads in the perfect spot and give the right pressure to mount.
The right service to do is to change thermal pads to reduce the memory temps, but if the card have warranty I don't see reasons to do if the card is under 100ºC
But your advice about GPU fan set and desktop airflow is good and can help reduce some degrees