I just finished this year doing both CHEM2915 and CHEM2916.
Basically, the actual content of the course and the exam/quizzes is exactly the same as CHEM2401/2402, the only difference in terms of standard content is that you'll do the advanced labs which are the same labs as CHEM2401/2402 but you just have harder questions to answer in the reports you write.
In terms of the SSP content, in first semester I did the Quantum Chemistry course and there were only about 7-8 of us out of the 20-30 SSP students who did it, and we all got basically above 95 in both assignments BUT if you aren't interested in quantum chem/very good at mathematics I'd steer away from the Quantum Chem and just do the other option in first semester. If you're interested in the quantum chem work just give me a PM and i'll send you some stuff.
In second semester, there's only one stream that all the SSP students do. It consisted of I think a course on materials (carbon nanotubes, MOF's etc), a course on drug discovery and molecular simulation. Each one consisted of a fairly straightforward assignment involving either writing a report on a topic related to that area, answering questions based on scientific papers, etc. All in all, it was quite easy and if you manage your time, not a whole lot of extra work.
Another thing, most of the SSP students will get basically guaranteed entry into the Summer Research Scholarships that USYD offer at the end of second year chemistry. They basically give you a scholarship and you'll spend 6 weeks full time work under a research supervisor (I'm just finishing mine up this week
).
So I'd strongly advise taking the offer that you got, I too was apprehensive at first but it's well worth it.