Extension work to a paper which is currently under review process : Is it good to communicate this?
I would suggest putting the new results into a new paper. There is no reason to notify the editor unless you plan to withdraw the current paper.

The current paper will/must stand on its own. If it does, then it will be published and the new results give you a new publication, perhaps. 

But if your current paper is rejected for some reason, then is the time to think about integrating the new results with the old, which really would tend to increase the value. 

I think the outcome is probably better if you wait and continue work, rather than creating a possibly confusing situation for the editor.