If I understand the results correctly, the throughput would increase from 100 images/s to 180 images/s, which is not almost constant.
Generally, you should see a better throughput by increasing the batch size.
However, the overall performance also depends on other potential bottlenecks in the model architecture as well as the training routine, such as data loading.
I would also recommend to try out the latest stable release, as some performance improvements were made.