Well we have kingtigers penetrated by fireflies frontally (ingame) ... so i think the SU152 will be worse -.-
HVAP from the Ordnance QF 17 pounder can penetrate the front of an Tiger II. And a 152 mm shell from the ML-20 howitzer will, if not penetrate, rip off the armour or cause massive spalling that obliterates the crew.
But yeah, you better hit on the first try or the faster guns will get the not too armoured I/SU.
PS. Theta; there is a ML-20 not three kilometres from where I live, shall I go and say hi? ^^