I doubt that there could be a definitive answer to the question; besides the Minnflier thoughts I could include that the attitude of the plane when it flies is also something to count. Some one have posted that he had successfully put the tank away several inches and the engine had performed well, even so, that does not determine the performance when the plane is flying because its constant change of attitude.
On some tank setups the benefit comes when they are almost sealed. Like the third line one pressurized with the muffler and a check valve (uniflow), but it will be a mess if the tank is in an upper position than the HSNV line. (You will find that the engine/carburetor is always flooded).
Like many things that we do ("when everything fails, then read the manual") testing is a choice but getting close with the manufacturers recommendations is always the right choice (no more than 3").
If there is a problem with your setup and the tank should be away from the engine, buy a pump and forget the problem.