Diastatic power is a measure of enzymes that convert starches to sugars. Malt extracts are created by mashing grains. Once the mash is complete, the enzymes are denatured during processing, so the extract has no diastatic power. I've never heard of using diastatic malt in pizza dough but pale malted barley, like 6-row, 2-row pale or pilsner are high in diastatic power.