I think Rubio still would have been the front runner in the early 2016 polling numbers if he made a more clearer stance on immigration. There is still tons of questions about the Democratic Senate immigration bill and Rubio's stance. If Rubio would have put the questions that were raised about his stance to rest, Rubio would have been in a much better position then what he is in right now. Rubio right now is following in Mitt Romney's failed path, which was allow your opponents and the media paint you as someone that you are not.
I don't think Rubio's Presidential ambitions are doomed. There is still a lot of time until the Republican primary kicks off. That time should allow him to do some damage control, the 2016 Republican primary debates should also give Rubio the opportunity to do some damage control. I also don't think his ambitions are done because he is a very influential speaker, much like Obama was in 2008. If you can speak well, you will do well.
Just for all of you know, I am not on the Rubio train for President. Right now I am on nobody's train, I wont decide who I will support until I see the actual candidates that joined the race.