Don't want to be the devil's advocate, but according to this
http://www.vcaa.vic.edu.au/vce/statistics/2007/section3/vce_specialist_maths_ga07.pdf you need pretty impressive results to get an A. Considering an A+ was 36/40- and an A being >32/40 in examination 1, i doubt 50% would take you anywhere near an A. But on topic, if you're capable, and you believe you can do it- by all means. However, i'd advise doing spesh if you don't have an instinctive liking to maths. the reason for this is- there are plenty of people out there that love their maths, and they will be achieving all the top marks. One more thing, thre is no use doing spesh if you plan to get ~25-30, because the scaling would only take it to ~38-41. IMO, if you spent that time on another subject (history or whatever), that feat could be beaten quite comfortably.