I have had the benefits of sailing on a lot of boats with sails from a wide range of sailmakers. There are definate differences from sailmaker to sailmaker and even from cheaper production versions of sails from the same sailmaker. These variations can have a lot of affect on how fast the boat goes, how easily it handles high winds or sails in light air, how long the sail lasts, and even how the sail looks after a few years of use.
The better lofts spend a lot on sail shape research, do more careful testing of the fabrics that they use, often have custom versions of standard fabric types with better UV protection, or fiber orientation. Very subtle differences can make big differences in the quality of the sail that you end up with.
I have found that lofts like Banks, Haarstick, Quantum and North who are involved in racing generally have better stress mapping which results in better shaped sails across the board. I have also found that timing my purchase to cooincide with big discount periods, such as boat show periods and slack periods during early winter can result in some big savings to the point that I have purchased top notch sails for the same price (or sometimes a little less and sometimes a little more) as the discount houses were quoting.
The other thing is resale. When I look at a boat with new sails by an off name sailmaker I immediately assume that they are junk and will need to be replaced. If I see a name brand I assume they were reasonable sails to begin with. Its just human nature.