How to Add extra clickable area to links and group

What other Spot values did you try?

Have you read the whole page Intro to Panels, not just about “Spot” Panels?

Regarding your other question, of course every additional element adds a tiny bit of overhead to the diagram, but I do not believe it would affect things enough to be noticeable.

Earlier I was setting predefined spot values,
Now I tried with custom spot values and that worked for us.
alignment : new go.Spot(0.5, 0, 0, 3.5), // top pointer
alignment : new go.Spot(0.5, 1, 0, -3.5), // bottom pointer

Thanks for your help.