Thanks for the info!
Weight is a pretty simple thing, actually - it is literally just the weight of one square yard of the material. (That's why I made my slightly smart-a$$ comment - a square yard of every 10oz fabric should weigh 10oz, regardless of thread count, quality, etc.). For this purpose, heavier cloth is better (until you get to the point where you're worried about too much weight).
Thread count is different - it's the number of threads per inch woven horizontally, plus the number of threads per inch per inch vertically. So a 600 thread count cloth will have 300 threads per inch, in each direction. This affects the 'fineness' or feel of the cloth, and the size of the pores/holes between threads. A 1000 threadcount fabric will have lots of very thin threads woven closely together, whereas a 200 threadcount fabric would have larger threads woven more loosely. For our purposes, high thread count is better, since we don't want large pores allowing hot gasses or particles through. I think one of the problems with the low-quality nomex I've bought on ebay was low threadcount- although it's a heavy (7oz) material, its also pretty porous.
I'm not sure a torch test is the best way to measure - for sure you don't want it to sustain fire (i.e., keep going once lit) but I think a torch is hotter and more sustained than what you will see from an ejection charge. I think most Nomex fabrics would burn through quickly if subjected to a direct blast from a torch.
