In my experiments using simply an amylase blend on cereal-mashed unmalted grains, I was never able to achieve an efficiency above 45%. My worts all came out thick and cloudy and fermented poorly, producing a variety of off-flavors and generally being poorly attenuated. When I switched to the Promalt (an enzyme blend containing beta-glucanase and protease as well as both alpha- and beta-amylase), my results improved drastically. My efficiency with Promalt is around 70-80%, depending on the grains, and my all-grain (or mostly all-grain) beers ferment as nicely as my extract batches. If you can achieve that with just amylase, I'd love to know the secret!