What is the impact high current through of narrow trace?
The same thing that happens if you make a thick chain with one weak link:). I would feel it was bad design practice, and based on your last question I would say either choose a better connector or use the outside rows for power.

Thinking about it a little bit more hypothetically but without doing any math if you're going to keep the current constant then your power dissipation would rise as that portion of the trace will have a higher resistance.   I guess if you held your voltage constant then you'd be able to deliver a little less current.

So I guess if your current is the same in all cases then a 20 mill trace will have the least power dissipation,  a 7 mil the most and your solution will fall in between probably closer to the all 20mill than the all 7 mil.

Assuming you have a reference plane underneath you're also changing the impedance of the trace at this neck down point since the capacitance will change here.  Although at the frequencies you mention I wouldn't consider this trace to be a transmission line.  But that cap change could be import to you depending on your application.