"Warrant" depends on how important editing / encoding time is to you. If the software you use can take advantage of the graphics card then even better. A 600MHz increase in clock speed is substantial when it comes to video editing / encoding because they are extremely CPU intensive.
I suspect the 2010 iMac has a 1st generation Core i7 while the 2011 iMac has a 2nd generation Core i7 CPU (Sandy Bridge). Sandy Bridge CPUs are roughly on average 12% more powerful than the 1st generation Core i3/i5/i7 CPUs. That means that the 3.4GHz Sandy Bridge Core i7 is equivalent to a 3.8GHz 1st generation Core i7 CPU. That is almost a 36% increase in raw performance.
If the graphics card can be taken advantage of, then the increase in performance is even higher. In short I would say, yes it is worth it. Especially since we are talking about a laptop which is pretty difficult to upgrade except RAM and the hard drive.