Unless the fuel tank is a good 12" higher than the engine, the typical .3 - .7" difference in height we see makes absolutely NO difference to performance.

It only affects the engine PRIOR to starting, in that there is a slight chance fuel will siphon in, particularly if you leave the throttle open, or if the throttle is closed but the engine out of tune ( overly rich permitting fuel to siphon in ).

Just watch your tubing. Keep lengths as short as possible and even with the tank inlet if possible.