This is a fairly loaded question, believe it or not! So, I'm going to be as objective as possible with this, and I've gotta say, whenever I see this question, my response is: the fact that you're even considering this probably means you should do specialist.
Firstly - further is quite competitive, and to be quite honest, the difference between a 40 and a 45 is quite often luck, but usually comes down to being meticulous and making sure to answer everything exactly as asked. Importantly, this means that you shouldn't make silly mistakes. So, if you're often making silly mistakes in financial, you've got a big problem - especially since it's part of the core units.
Now, let's move over to specialist. Yes - maybe you need to catch up. However, specialist 1/2 really has nothing to do with 3/4, with the exception of some very minor topics. More importantly, to do well in specialist 3/4, you'll have a solid base in methods 1/2 (or even 3/4 if you've done it), and score well in methods. The only risk is that specialist is harder, and plenty of people who did really well in methods didn't do so well in specialist. Having said that, plenty of people I know did really well in specialist, but not so great in methods, so this is one of those things you have to experience to believe.
All the said, from the sounds of your situation, I'm going to assume that you're a decent maths student, who will work hard throughout the year, and in Further, you will get a 45 study score. That's great - BUT, to compare the two maths subjects, we need to look at scaled scores. I will use last year (because that's what our own ATAR Calc gives me) as the basis.
So, if you did these subjects last year, you got a 45 in Further, this is a scaled score of 44. To get the same score in specialist, you only needed to get 33. That is - to get the exact same increase on your ATAR, you only needed 33 instead of 45 in your subject.
In my experience, if you're considering a 45+ for Further, and at the same time think that specialist might be a worthwhile subject, then 33 is easily achievable with the same amount of effort - and you might find you do even better. So, I would recommend doing specialist.