The chief issue is our altitude here. That limits the maximum amount of vacuum you can pull. At my altitude (7000′), I can get just a hair over 26″ of Hg on the vacuum gauge when I get everything really tightly sealed. At sea level I expect I could get at least 29″. At sea-level, that would be a bit over 14# per square inch.
If you really want to make tight plate, you can do better by making a press out of multiple layers of 3/4″ melamine board and parking the front wheel of a decent sized pickup truck on it.
how much real difference is there in the final product if you go from say 10″ to 20″ of vacuum?
Not that much. As you pointed out normally I draw as much vacuum as I can. However, there have been cases that I intentionally bleed off and lower the vacuum. Typically this is when the possibility of crushing or distorting a part but having too much vacuum.
Doug is right, the main difference would be the amount of epoxy forced out of the fabric into the breather material or sucked into the epoxy trap if you use one. Lower vacuum pressure reduces the compression on the layers and leaves more resin present in the layup so ideally you pull the highest vacuum you can manage. The ideal resin/fabric ratio is about 45:55 % by weight. Basically you want just enough epoxy to thoroughly wet the fibers out and not a drop more. The resin doesn’t provide the strength, the fibers do. All the resin does is lock the fibers into place relative to one another – keeps them from sliding actually.
Also as Doug indicates, it all depends on the application. Some projects demand much lower vacuum pressure to prevent distortion or damage to the piece.