If and only if you have a backup heat sink to test, I would consider drilling holes through the existing heat sink fins in the direction the car travels…perpendicular to the fins.
I’ll tell you a story which will probably seem irrelevant, but actually is related (indirectly) to this. A long time ago when the first aircraft were being built for later WWI and earlier WWII, there were bigger and bigger engines. The cowling around the engines were known to help flight with lower drag, but they also thought perhaps this was restricting flow of air and increasing engine temperature. They experimented with removing the cowling and exposing the engines directly to the air flow, but it didn’t turn out as they thought it would: The engine temperatures actually went up when cowlings were removed. What they discovered was that if air flow is directed across the surface of the engine through the cowling that the smaller air flow with a directed path exceeded the cooling ability of the increased total air flow without the cowling.
How fast is your car moving? Is it moving most of the time? Would you be able to go to a lower power mode when stopped, and only increase clock speeds and performance modes as the car moves faster? Do you have a way to direct air flow (assumes the care moves fast enough to have some sort of flow)? How hard would it be to rotate the TX2 itself 90 degrees so the fins line up with the direction of air flow?
If you had a controlled air flow, then you might be able to remove the existing fan. Even with the fan, increased air flow might mean you can drill holes in the aluminum fins to lighten the weight.
Now also imagine simply moving the fan with intake from the forward direction and exhaust towards the rear. Not exactly propulsion, but it would counteract the air pressure slowing the car while adding a benefit to cooling. A correct intake duct might partially take advantage of that (a tiny tiny advantage with a fan that small, but you said you are interested in even tiny improvements).
