From my experience the jump between the AAT and CIMA is a big one - the guide on hours per paper I believe is 200 hours per paper.....but the biggest differences are in exam presure and techniques.
Its the same as anything, if you put the hours in you'll get the results - I did AAT and found it (as many do) useful but rather easy....I then completed CIMA and put in 100x more work to do so, for the final four I gave up four months to it and did nothing but study (aside from working, I mean no social life)....but I did get first time passes that way.
Good luck with it - Choose a good training provider if you can afford it/have study support.....and work on exam technique and time management from the first paper you sit.