You will experience a pro, well ported 50cc saw the way it was meant to be when you move from a 20" bar to a 16" bar.
It's gonna rip!
Increasing chain speed with a bigger drive sprocket on an already fast chain (and a short bar with almost no resistance to kick due to being short and lightweight) will kick like lightning if you don't thoroughly respect the top corner of that little bar. I would try it as is before going to an 8 pin. I doubt that you will want any more speed.
Hey, it's pretty much all about the chain tuning anyway.
Let us know what you think of that saw with the shorter bar. I bet you will discover the raging animal within..
I have some nice 50cc saws, all with 16" bars. Every one will sink bar deep in almost anything without even noticing. I am getting tired of stooping tho...I may put an 18 on a couple of them soon.