We have the larger CS-510 (2) and it is maxxed out with an 18" bar, even though Echo rates it for up to a 20" bar. We also had a CS-440, the next saw up from the 400, and it was not comfortable with an 18" bar, did OK, but really shined with the 16" bar.
Echo saws make decent power, not overly impressive anyplace, at least the ones we have tried, aside from the older reed valve engines, which cut WAY too slow for my liking.
We keep hearing that all of our Echo saws are going to develope some sort of "death rattle". Interesting that we have an Echo garden tiller, leaf blower (2), string trimmer, and just about every saw in their line-up, and haven't had the first trouble with any of the engines. We have actually logged many hundreds of hours on our two CS-510's, one of which is loaned out to a friend who heats his home and shop with firewood, and cuts about 15-20 cords of wood a year with it.
We did find with all of the Echo saws we aquired, that the carburetors required custom settings, they were WAY too lean as delivered, which probably would have led to early engine failure? In any case, ours all went lean in the cut, so it was obvious that they needed more fuel.
I also opened up the mufflers on a couple of them. It didn't make much difference for the CS-510, but a dramatic improvement on the CS-360 top handle. Your CS-400 if newer production may use a catalyst muffler and respond well to opening it up some?
Echo also uses very "low" quality chains, we quickly burn them up and put real chains on them. They respond by cutting faster with less effort. I would run the 18" set-up till the chain is toast, and replace it with a 16" bar and real chain....FWIW.....Cliff
Then, when you run your saw, strap on your PPE... 
