I'm doing IT Applications without Units 1 & 2 this year.
Well I did do some of the excel stuff for Unit 1, as I started off the year doing Unit 1, but the school decided to move me up into IT Applications in the first couple of weeks (luckily I got in before the deadline for changes to enrollments).
So yes, it is do-able without Units 1&2. I've been doing really well in my sacs (I've dropped 2 marks the whole year - excluding the U4O2 sac because I haven't gotten the results back for that). So yeah, I haven't been in anyway disadvantaged by not doing year 11 IT. Of course, how I go study-score wise will depend if I screw up the exam - but that wouldn't be due to my lack of not doing Unit 1&2.
The subject is pretty easy. The hardest part is staying motivated enough to do the work. There really isn't anything in the course that I've found difficult. Web design is pretty simple - HTML and CSS is pretty simple (I threw in a bit of Javascript as well). Databases are fairly straight forward and spreadsheets are easy as well. Some of the theory stuff takes a bit to get your head around (I couldn't get my head around Normalisation at first) but most of the theory really isn't technical stuff anyway.
The SACS are long and involved. You have to plan out your time and make sure you stick to it during the sac. The theory segments of SACS are easy. You can think over stuff between the classes as well, as the sacs do go for at least 5 to 9 sessions. I went through most of the year without doing all that much homework (class time is pretty relaxed for IT). The exam is really the only reason I was prompted to finish the textbook and do other study. I feel kind of guilty for neglecting the subject, but I reckon I've managed to achieve a good understanding of the course.
To be honest - I don't like IT Applications. It's a very dull course. You do website design and databases in Unit 3 and then we did spreadsheets for U4O1 and the final outcome is theory questions on computer security and ethics. I'll be doing Software Development next year (managed to convince enough people to do it) and if your school offers it, I'd try to do that. While it still looks pretty bland, it definitely looks to be a better and more challenging course than IT Applications.