That's sort of the idea, not only is Luke in agony but the spirits of his dead mentors are also in jeopardy.

The scene should be horrific.

As I said earlier I'm not sure if it would work but part of the problem is this Force Ghost business was pretty much made up at the eleventh hour of shooting ANH and even after all these years has never been thought out properly.

Just as Obi-Wan allows himself to be killed he claims that if Vader strikes him down he will "become more powerful than...(he)...could possibly imagine" all Obi-Wan does in spirit form is give Luke a bit of shooting advice, direct him to Yoda and confess to fibbing a bit.

He could have done that with hiding a letter in Luke's pocket (Marty McFly didn't need to be a ghost to save Doc Brown).

While I wouldn't go as far as bringing them back to life and physically joining in the fight (as in one of the dropped elements of earlier draughts of the script) it makes sense that their spirit forms wield some power beyond that which we see in the theatrical versions.