Spike might be able to answer this--I think he has a 257 in his collection and has been running/breaking in a 555.
Bottom line, if you want to pull 20" of 3/8 full chisel with authority and the ability to lean on the saw with much lower risk of chain stoppage, you're in 70cc territory. It's an inconvenient truth, for your wallet at least.
I think the 18" setup is wiill prove suitable for these saws. If you talk to a serious saw shop, even with the 359, 361, if you really wanted comfortable and powerful cutting, they would definitely set the saw up with an 18" and tell you that 20" is optimistic.