I am trying to decide between the Canon t4i (650D) and the t3i (600D). Initially I was going for the t4i because of the autofocus. But the sample videos I have seen did not really impress me.
Can I expect improvements from a future firmware update or are the limitations mostly through hardware?
The 650D/t4i (like all modern DSLRS) has two separate auto-focus systems:
The "phase detection" system is used when using the optical viewfinder and the "contrast detection" system that is used when using the camera's screen (and when doing video since you can't use the optical viewfinder for video).
The new feature in the 650D/t4i system is an added phase detection element to the contrast detection system (what they call "hybrid autofocus"), I'll still refer to it as "contrast detection" in this answer.
"phase detection" systems are very fast, "contrast detection" system are typically slower but more accurate.
The "phase detection" system of the 650D/t4i is very good and very fast.
The "contrast detection" system is not as fast as in mirrorless cameras (that don't have "phase detection" at all) but still very fast compared to other entry level DSLRs.
So, can a firmware upgrade improve contrast detection auto focus? probably yes, contrast detection is mostly software.
Will a firmware upgrade improve contrast detection auto focus? very unlikely because right now the 650D/t4i (with an STM lens) has the fastest contrast detection auto-focus system in the entry level DSLR category !!!!!
Can you expect an upgrade? I think unless an actual autofocus bug were detected the answer is "highly unlikely". From past history, the autofcous has been, as far as I am aware, only been improved when it was found to contain a bug. Unless I am mistaken the 1D MK III was infamous for its AF at the start...
On the other hand - as was pointed out, you are left with pure speculation as nobody anticipated firmware 2.x.x for the 5D MK II that added features, video features, but extra features nevertheless.
