STEP early

Hi. I'm currently in year 12, and am going to be applying to cambridge in y13. I know that STEP scores from previous years aren't counted, but if I get a good score (1,1 or better) at the end of year 12 can I put it on my PS to show passion and boost my application?

It shouldn't do you any harm - provided you are confident of getting (at least) a 1 - anything less would suggest you're just trying it early without being properly prepared. The trade-off is that if you're spending time on STEP (which is difficult), you don't want to be risking neglecting your A levels - hopefully you've taken this into account.
I wouldn't say doing STEP demonstrates "passion" for Maths, but anything that extends the curriculum or shows that you're prepare to tackle more difficult problems or ones that require more concentration than A level work are a good idea - could be STEP, AEA, competition stuff if you're into that (not everyone is!), or similar

Hi. I'm currently in year 12, and am going to be applying to cambridge in y13. I know that STEP scores from previous years aren't counted, but if I get a good score (1,1 or better) at the end of year 12 can I put it on my PS to show passion and boost my application?

No, don't do it - you may have to resit in Year 13 anyway and a weak result will have to be declared.

I wouldn't be doing it if I didn't know for a fact I could get 1,1 or better

No-one can guarantee they'll get a 1,1 - remember 50% of offer holders miss their STEP requirement.

I'm aware, but as I've said, I've got like 8 months and I've already got 1,1 on a few past papers. And that's with little preparation. With the extra 8 months, I have no doubt I'll be able to get S,S on at least one and at the very least 1,1 in the actual exam