Nope it wouldn't pay of as Aggron's Speed Stat is low, unless you use a Rock Polish set.
Personally though I would recommend one of these Natures: Careful (+Sp.Def -Sp.Atk), Adamant (+Atk -Sp.Atk), or Impish (+Def -Sp.Atk). If you're using a slow set (for Gyro Ball) have Brave Nature (+Atk -Spd).
