Has the car ever run on track without it overheating, or is this new behavior?
Compression test only tells you that you have a good seal at the rings and valves. Pressure test on the cooling system will only tell you that there are no coolant leaks, it doesn't tell you how much flow you are getting.
You are getting enough coolant flow to handle the heat load generated when idling. When you run on track, you need more coolant flow to overcome the additional heat load.
Bad water pump, collapsed/blocked heater hose, partially plugged radiator, limited air flow path in/out of the radiator spring to mind. We had a radiator that got partially plugged and exhibited that exact behavior (ok at idle but too hot when running on track). We diagnosed it with an infrared thermometer pointed at the radiator surface, we saw different temperatures across the face of the rad (cooler where it was plugged because no hot water from the engine was getting to that part).
We swapped radiators and the problem went away.
Do you have any spares with you? The above items should be easy to replace and check (if you have any). If you have a blockage internal to the engine, not so much.
We Audi Be Faster
'85 Audi Coupe G(in &) T(onic)