maths at uni is mostly new content, so it's not as if when you get to uni you will have to "cram" everything in extension 2
however the additional skills you learn in extension 2 maths will make uni maths easier, and also it does mean that you will find complex numbers a breeze in first semester. some other things will pop up at uni which extension 2 maths covers, like integration by parts.
also, expect physics at uni to be a lot like the mechanics topic in extension 2, so it's worthwhile learning that section of the course.
so if you are capable of doing extension 2 maths now, then it is worthwhile making the effort - but obviously if you are feeling overloaded already or are not enjoying extension 2, then don't stress yourself out by keeping it.
good luck for the hsc!