Hi Tk,
Wall of text incoming.
I did my YOU and specialised testing just a little while ago, so this should be pretty up to date. I'm no maths whiz, and I've been out of high school for more then 5 years with no sort of maths study/use in-between, and I got through. So no, you don't need to be some kind of maths whiz.
For the specialised testing, the math involved was either fast mental maths, or somewhat more involved physics style questions. They certainly won't be asking you any calculus questions. Maybe I got lucky, but I decided to give the courses a miss, and just focused on re-learning my times-tables (!) and used some of the various resources on aviation style physics questions in this thread to prepare. That seemed to get me through ok.
That said, while I passed, my results were not great (valid for one year), so to make yourself more competitive you may want to study up on that maths. I found the physics style questions especially hard.
As for your question on the importance high-school results and your current results... Honestly, who knows what the recruiters think? All you can do is do your best in your current maths courses, and go for it. Personally I think wondering about how competitive your results will make you is a waste of time, and if you're truly committed then just do your best, and don't get put off by any set-backs. I guess that's somewhat cliche, but I think that's how it is.
Good luck!