Don't try to be cool and buy something 15 levels above you because you'll get less stats from it (by a lot).
That, or you'll get more stats from it than a piece of equivalent level:
I wore a Rank 21 tunic at Rank 7 and it only gave me ~27% lower defensive stats (still better than my rank 1 fodder; it also varied by stat) and the full DEX bonus
(+3), which could never have been given by any at-rank piece.
Test these things yourself: it is (for some crazy reason) different for each individual piece of armour (I've seen decreases of 24% all the way to 54%!); at this point in the game it's pretty hard to say, without extensive stat-comparison and testing, what is actually better than what.
Furthermore, the higher-rank piece had the additional benefit of allowing me to "grow into" it. In short, I would not outright decry wearing somewhat-significantly higher-ranked gear, it's a weird
system, and I can only hope it changes drastically. ¯\(°_o)/¯