I think they did a really amazing job of making Ultron sound like Tony, both speech patterns and sound-wise. To the point where I'm wondering if they did any post-production to loop RDJ doing some of the lines.

I think it's also that Spader had to do most of the heavy lifting with just his voice, because, unlike a lot of the mo-cap character, Ultron has to move like a robot so he can't use body language in quite the same way.