Conversely...

At 84th, you might be mostly wearing a combination of level 80 raid/pvp gear and/or whatever quest rewards were improvements over your level 80 gear.

At level 85, as a fresh toon, the rating conversion sucks initially and you might even be worse in the same gear than you were at 84th.
But you should acquire level 85 gear, which will eventually make you far better than you ever were at 84th.
The trick is just getting there.