The sailmakers you mention have their sails built in China but they use
another loft, one that I am quite familiar with. In fact Nigel Calder
wrote an in depth article about the loft for a British periodical. I'd
be happy to send a copy to anyone interested. The article even names those sailmakers.

Teijin is not well regarded by most sailmakers. Lee Sails Northeast advertises on their website that they use Challenge and Bainbridge.It's a far cry from the higher end cloth from Dimension Polyant, Bainbridge, Challenge, and Contender. It is cheap though.  I've used Teijin cloth on occasion for some small boat projects like a set of sails for a 13 foot daysailer that is  used by children for daysailing on a lake. There are several lofts using Teijin produced cloth for small boat and Hobie Cat sails. It allows them to keep the costs way down. For instance there is a supplier here in the US that sells Laser sails for about $170 USD that uses Teijin. Another loft sells Hobie Cat sails starting as low as $400 or so. It is an appropriate cloth when cost is the primary consideration.

The amount you paid for your sails is less than the cost of cloth and shipping finished sails from Asia to Europe or North America. If Teijin put out a good quality cloth that was comparable with the manufacturers I mentioned, reptuable sailmakers would be using it in quantity.

Caution is in order for anyone ordering direct from Lee Sails in China. They require payment by wire transfer or bank check. You should never buy anything like a sail unless you can use a credit card as there is no recourse in the event of problems. You can contest a charge made with a credit card successfully. I had a dock neighbor who tried to save some money and ordered a sail from a loft quite some distance away. The sail was a disaster and when the sailmaker failed to address the issue he was able to contest the chrage and get his money refunded.