Not as much as you may believe.

Hold the nose of your plane up while tuning and you are adjusting the carb for flow when the tank height is a good 8+ inches BELOW the engine.

The plane leans a bit, but it's not all that dramatic once the engine is properly adjusted.

The relatively LOW amount of difference when mounting an engine inverted hardly affects an engine at all.

If it did we could not fly uplines.

If the offset was 12+ inches then there MAY be issues with the weight of the fuel in the lines, etc...

However the often touted "tank height" difference affecting engine performance is largely a myth.... much like the "fuel stays in the back of the tank" myth on downlines... often repeated by even VERY experienced people who never bothered to empirically test this... I can't believe Clarence Lee said that in his articles... ugh.