M1a1 aim

Why is the first and only APFSDS shell on the AIM a tier 3 upgrade ? Why is my first “better” shell a proxy heat with 350mm of pen ? Is the AIM supposed to be a helicopter hunter for the first 50-75 games ? It should be at worst a tier 2 upgrade considering the fact that this thing faces off hordes of ERA covered T72Bs and T80s