You can get an ATAR of 98 with any combination of subjects.
If you are struggling in Advanced, changing to Standard won't make a difference unless you address what you are struggling with because they are pretty much the same subject where the only material difference in the difficulty of the texts.
However, I doubt there are many students with an ATAR of 98+ who also did Standard.
If you could get band 6 in standard, you most likely would have gotten a higher mark in advanced. Advanced and standard scale exactly the same. Its just one is harder so they give u a higher nominal mark (advanced) for the same exam marks. Standard has easier to understand texts and easier questions but because of this it is harder to get a higher band.
I wouldn't recommend dropping Advanced, instead, I would insist that you continue advanced; no point in dropping it (like BLIT2014 said; "it doesn't guarantee that you'll get top marks in Standard").