The flat smile shaped airdome diaphragm service a few purposes beside isolating the suction and pressure side of the pump. It is meant to give into the dry chamber when the plunger comes down generating the pressure stroke. This flexing reduces the max pressure at the beginning of the stroke, and toward the end of the stroke as pressure is reaching it's lowest point returns to it's original position which smooths out the pressure pulse and extends the movement of fuel out. A trick to raise pressure on a pump with low output pressure would be to double up that diaphragm. Or reduce it's inward flex with the hard sponge.