It does look a little "cleaner" without the vents?? but if I had to fly it, I'm not sure I wouldn't want to keep them, if I were making some "hard pulls" down in the low end, they are nice insurance and give your aircraft a forward pitching moment built in.
Some of the early century fighters would get into a "stalled out" condition, with an aft CG from which there was NO RECOVERY??? that's something you will not have to worry about as long as those ventral fins are down there in back, actively "pitching the aircraft" nose down when you are in that very low speed or even post stall mode!
They are not even noticeable in normal flight regimes, and do give it tremendous yaw stability?? if you have ever shot an arrow with those little bitty vanes, the arrow will oscillate for a much longer period than if you have those nice large Feather's. Which quickly "damp out" those energy robbing oscillations, in archery you get much better shaft penetration, because you retain more energy.
In aviation, you get an aircraft that recovers quicker, and that you don't have to "sweat" when you get slow, Dr. Song and team, incorporated those vents into the J-20 for a reason??
