Obviously gains fall off as you climb, not to mention draws and losses hit you for more points. I don't think it's reasonable to expect him to reach ratings that are even above his performance ratings.
If he were regularly turning in 3100 and 3200 performance ratings, then sure, it's possible... but really 2900 is amazing enough. Lets take this 100 points at a time
At one point, I thought 2900 was impossible, even for Carlsen. Now, it looks like he may break it in the next few months!
Having said that, 3000 really is out of reach, even for Carlsen.