All stations get 19.2 Mbps in 6 MHz of bandwidth to use. But you are correct in that if you have a weather sub-channel that is relatively static, you can use less bandwidth for that channel and more for the others. I have noticed that the PQ for ThisTV on KSLA 12.2 Shreveport is much better than that on KLTV 7.2. I suspect that this is because KSLA's other sub-channel is Stormtracker weather, and they can get by with less bandwidth than LaVida on 7.3.
Hopefully KSLA will continue with ThisTV so those of us in the eastern part of the Tyler-Longview DMA can continue to receive it. But, if this is a decision coming down from Raycom Corp KSLA may be replacing it with Bounce as well.
At some point London will be doing something with KCEB, so the opportunity for additional sub-channels in the future still exists. KFXK and KETK still have room for 1 additional sub-channel each, and there should also be build-outs of digital facilities for KTPN-LD and KLPN-DT.
Shreveport has 3 stations with no sub-channels at all - KTAL, KMSS, and KSHV.