Well, just out of my mind - 122mm could wreck 60mm plate(like IS-2 did with Panther turret side). That's not taking into account weld lines and whole set of outer equipment like sights.
Do we have good estimates available for penetration of various impact fused HE rounds ?
Well, those would wreck most tanks anyway, difference wound be only between FU and FUBAR.
Plunging fire from 122mm HE or larger will go through as well I think.
Plus threats different from tank cannon fire, like top-attack EFP... May look at comparison of top protection of M1 compared with T-series. Highly angled steel plate is really compromise decision.
I think another potential problem is that even slightly plunging fire is going to be a real problem if you are relying on a high slope to deflect the shot.
But this is not so comparable to lower velocity plunging indirect fire.