The 128mm used 2 part ammo (IIRC) and compare with the soviet 122/152 in terms of explosive filler. It packed a punch. The Elefant used a long barrelled 88mm. The 88mm HE was a good AI round (better than German 75mm HE), but it wasn't in the same class as the Soviet 122s.
I think it is likely that the Jagdtiger would be a merging of the Elefant and the ISU-152. It would probably be even slower than these two vehicles, as well.
The ISU-152 is an Assault Gun mounting a 152mm Howitzer, the Jagdtiger is a Tank Destroyer mounting a 128mm Anti-Tank Gun.
Besides the ISU-152 isn't exactly a key picture of balance at the moment...