Yes, that'd likely result in a very rich condition (too much fuel, not enough oxygen) - which would be exasperated by running at altitude (even less oxygen).
Although it seems the opposite of people reporting having issues with not enough fuel moving through?
The Espar pump is a metering pump, so the problem is not that it's too rich - actually it's probably the opposite problem, the suction negative relative pressure of the pump causes air bubbles (vaporization/cavitation) due to the high vapor pressure of gasoline, and this problem gets worse as altitude increases, causing no gas to get to the heater. The 2mm (vs 6mm) stand pipe helps a little bit by having lower cross-section so that the surface tension of gasoline (small as it is) can help reduce vapor lock (vaporization/cavitation).
As I mentioned above, this is a physics issue that applies (in different measure) to all fluids. For example, with water, the theoretical maximum vertical height (head) that a perfect vacuum pump can accomplish at sea level is about 33 feet, due to water's vapor pressure of 1 atmosphere at sea level. At 6000 feet altitude this theoretical maximum lift decreases by 33% to 22 feet. Putting a pump any higher than that (above the inlet of an open air tank) and the water will begin to vaporize. In practice, due to friction, less than perfect vacuum, and other factors the practical limit is 22 feet at sea level (or for a centrifugal pump, even less at about 15 feet), and again it will be 33% less than that at 6000 ft, so about 15 feet (or 10 feet centrifugal). To be clear, these "head" heights refer to the vertical distance between the inlet (suction-side) of the pipe and the pump itself. So for a well, if the pump is located at ground level, it won't be able to pull/suck the water up from the well shaft if the inlet pipe to pump height is greater than that.
Gasoline has a vastly higher vapor pressure of about 9 to10 psi, which is about 40X more than water. I'm not exactly sure what the maximum lift of gasoline is at sea level, but it's likely not more than about 3 feet. And at altitudes higher than about 12,000 feet, standard gasoline will start to boil, so that the maximum pump lift is zero. No pump from any manufacturer is going to work pulling gas out if the gas is already vaporizing, because the pumping action is now merely serving to increase boiling/vaporizing.
The point is that due to gasoline's high vapor pressure, high altitude operation of the Espar, Webasto, or other heater is going to depend in part on the fuel line and pump configuration. The most critical factor is minimizing the vertical height from the standpipe inlet to the pump, but reducing the tank stand pipe-to-pump line length, reducing the number of tight elbows, using the 2mm standpipe and tube, and orientation of the pump to between 15 and 30 degrees above horizontal will also help reduce the tendency to cavitate/vapor lock the heater.
Adding kerosene or diesel will also help, because their vapor pressures are both significantly lower than gasoline (kerosene is more than 50X lower!), but obviously this may have unintended consequences for the engine.
For example, kerosene (and aviation gas which is very similar), boils at about 125 degrees C at sea level (vs. 100 degrees for water). This explains why @Chainman1
's heater started working when he put kerosene in.