Depending on your platform/device, an atlas has a max size of 2048x2048 or 4096x4096. Trying to pack anything bigger than that will just flat out fail.
The effort involved in packing at run time doesn't really pay off enough in performance if you only save <50 draw calls, aif you go from 51 to 1. If you go from 150 to 100 it might be a different story, but generally, it's not really worth it.
40 draw calls will generally not do anything, but if you get into the order of hundreds, you might have to start looking at it.