I would like some clarification please.
So if I have a 15|X creature, it would attack for 15+30=45 per turn for 2 turns before dying?
If I have a 16|X creature, it would attack for 16+16=32 for 1 turn before dying?
At 15 attack, it will take two turns to die. For each of those turns, the shadow stalker's effect is as if (though technically not) doubling its attack.
So, turn 1, it would deal 30 damage, and 30 again the second turn before dying.
You are correct about the 16 attack example.