Background: I actually have a degree in IT.
I think the important thing here is to think about whether you want to go into IT or Computer Science first off. They are two different areas, even though they sound similar. I like to make a very easy distinction between IT and Computer Science. IT is the application of existing software/hardware etc... it usually has a business purpose. So you may find that IT peeps work in support or touch on development, database admin, system-based things. Computer Science is the one that requires the maths, because they focus on areas such as the logic/algorithmics behind compilers, they program, they look at AI (just thinking of a few examples off the top of my head).
Anyway, now we've got that out of the way... Majority of Computer Science courses require methods as a prerequisite, so you've got that covered. Personally from the information you've given in this thread, i'd say do Spesh purely because higher-level maths will benefit you in the long run. I got through fine with just Further but if you want a stimulating challenge, then do Spesh.
Differing from the others, given that you're interested in IT, maybe algorithmics is something you could consider as well- here's a link to the study design if you want to take a look: http://www.vcaa.vic.edu.au/Documents/vce/algorithmics/AlgorithmicsSD-2017.pdf
Second this. Algorithmics started in 2015 and is a HESS (higher education scored study). It scales around the same as Methods due to the similar difficulty. You will most likely have to do it through DECV unless your school is one of the select few that offers it (there's around 18-19 schools now that offer it as of this year). It is by no means easy, though... so you'll have to work for the high SS. It's about the same as a 1st-2nd year CS sub at uni.
I don't think you'd be significantly disadvantaged if you did Further either, to be honest. Some of the areas in Further are beneficial for IT (e.g. networks, recursion). Given you already have done methods, that's imo all you need.
Do you have a specific area in IT that you're interested in? IT is a very broad discipline containing quite a number of areas.. maybe this could help narrow the decision down for you.