agility does not effect sneak in this game
sneak perks are the only effective way of getting better (edit: apart from +sneak gear ofcourse)
weight and armour are irrelevant too, only movement speed while sneaking makes a difference (which rank 4 negates)
also worth noting having rank 2 & 3 (no trigger floor traps and mines respectively) works whether you are sneaking or not
there is nothing listed anywhere about agility effecting sneak
so if it does there will be no way of knowing untill GECK comes out