Diminishing Spawner Rates
Stacked spawners now have diminishing returns for how many mobs are spawned:

Stack Size: Mob Spawn Multiplier (+Increase)
1: 100%
2: 150% (+50)
3: 200% (+50)
4: 240% (+40)
5: 280% (+40)
6: 310% (+30)
7: 340% (+30)
8: 360% (+20)
9: 380% (+20)
10: 390% (+10)
11: 400% (+10)
12: 405% (+5)
13: 410% (+5)
14: 415% (+5)
15: 420% (+5)
... continues with +5% per spawner until stack size limit