Ok here's my two cents on the topic.

I think it actually makes sense for The Emperor to call him 'young Skywalker' because a part of him will be pissed off over Luke blowing up his Death Star (that took many years to plan and complete) - and therefore 'young Skywalker' screwed everything up for him.

I hope that made sense; because it makes sense to me :)

I have no problem with the original line btw, but I actually think the new line sounds more powerful. It may be almost impossible to create the original line seamlessly just with McDiarmid's voice, but we'll see what happens.

EDIT: Oh and Vader's line 'If he could be turned....' comes after the 'young Skywalker' line of Palpatine - which makes the new line make sense in a way because Vader is obviously concerned about the welfare of Luke rather than thinking of him as a potential enemy like the Emperor does.