How to handle remote validation Add and Edit form

performance has nothing to do with model.

more over models are of two types Business Model (pure business Entities) and UI specific Model (View Model)

if different view requires different properties of same class then we should create different view model.

if two views are exactly the same that same model can serve it then no need to create a new model.

If in Edit view you don't want the remote validation then create a new EditVM , each view should have different view models untill two views are not exactly the same.