I assume the Series S will use 5GB of the 8GB for video. Having half the video memory when pushing half the resolution actually makes complete sense, and is right in-line with how video settings currently work on PCs.
As for the memory bandwidth, that number is exactly what the GTX 980 has (224GB/s), more than the GTX 1060 (192GB/s), and within spitting distance of the GTX 1070 (256GB/s). Once again, considering the Series S has an RDNA2 GPU, I imagine it could get close to GTX 1070 performance, which is an incredibly capable 1440p 60fps card, and also did great at 4k 30fps on Xbox One tier games. The main difference being the 1070 was originally designed to also run at 4K, hence why the memory bandwidth is likely not as necessary at 1440p.
To cap this off, the Series S will only be pulling half the amount of mip-map memory as the Series X, yet it has the exact same SSD and texture decompression grunt. It will be a very capable little machine, and probably especially perfect for people who care more about framerate than resolution.