It's unlikely a standard line doubler will do much for you as it really does not "double" the lines, it only de-interlaces them. Since you already have progressive scan--it's not going to do anything for you (some doublers do not accept progressive input). The Faroudja native resolution might help the picture quality significantly. It uses a fair amount of post processing along with a scaler (as opposed to a doubler) to improve the picture quality--of course you pay for it--Faroudja is not cheap--but does make great quality pictures.
LIne doubler/scaler
I heard that if I was using a high quality progressive DVD player, with a DLP projector, using a line doubler will not likely make much difference in the picture. Does anyone know if that is true, or would a line doubler give me even a better quality picture. Along the same lines, DVDO Pro or Faroudja given the price difference between the two. Thanks.