According to my sources there are several suppliers of Bruce Modern tartan in a variety of weights in wool:

Lochcarron in 10 and 13, Batley in 13 and 16, House of Edgar Old/Rare in 13, and Strathmore in 11 oz.

From your tale so far I assume you are desiring the Batley in 13 (logical, inexpensive but decent tartan) but they are offering to substitute the Batley in 16 for the same price but a longer delay than desirable.

First, I doubt that ANY retailer would be able to turn around a new kilt order before early March right now with the holidays upon us, unless you pay the rich premium for a rush job. The best shot at an earlier delivery is with the current vendor, who you may be able to guilt into a rush kilt build in whatever tartan you end up with.

Second, I assume you chose the Batley for throwing because of its low materials cost, and the 13 oz because of its comfort level. Seeing as Lochcarron and HOE also offer the same tartan in the same weight (and probably better colors IMHO), and Strathmore in 11 oz, you could ask the vendor to instead source one of those tartans for the same price, or at least a significant price break on the difference in materials cost.

Third, although I must admit I do not do "heavy lifting" exertion in my kilts so cannot equate my feelings with yours in that regard, I do not really notice the difference in weight of tartan so much as the amount, so a 4 yard casual in 16oz sooner than a 13oz later (potentially much later) would not bother me. And if my memory suits me what Batley calls 16 oz really did not compare to other mills' 16 oz the last time I had some swatch books in my hands, so may feel lighter and cooler than you expect. And the 16 oz will likely wear better than the 13 from a kilt longevity standpoint.

Good luck Pete, hope something I added helps, even if only a little bit.

jeff