Couple things. How warm does 14 ounces wear? My tailor-to-be suggested that I will wear a coat over the sport jacket most of the time, so weight is not that essential. Is it aesthetics more than anything else? A bit about nomenclature as well. What makes a cloth tweed? In other words, I just looked at a swatch online of blue herringbone. It is 100 percent lambswool. Normally I would associate herringbone with tweed. But is this something else? I thought earlier that you had distinguished lambswool from tweed. Maybe I'm splitting hairs. Sorry if this is an obvious or stupid question.
Everyone is different with how things wear. I find 14+ ounces to wear reasonable warm, such that in 50-65 degree weather I can comfortably wear it without an overcoat. Depending on climate, this means per your tailor that you will have an overcoat on as well. Weight is thus mostly relevant for the range of temperatures you can comfortably wear something without a coat (the top of which will of course be the warmest temperature you can wear the jacket in). You also have a lot of fabrics that offer tweed-like patterns but are not themselves tweed. Tweed fabrics tend to be heavier and rougher. I am sure there are other defining characteristics but those are the ones I tend to think of.