No one claimed these two examples are equal, at least not me. What I meant is that the DPS (measured by the T2K) of the ISU against heavily-armored targets is roughly equal to that of the SU-85. This of course includes other factors than pen, such as ROF or the fact that the ISU deals deflection damage, which diminishes in effectiveness the higher the pen chance gets. The latter is the reason why penetration bonuses are less desirable for the ISU than for other tanks, and an equal increase in ROF would be much more beneficial.
I vaguely remember that we once estimated the ISU's AT capability to be roughly equal to an SU85. So overall, it should lose to a Panther.
I might be misunderstanding hannibal here, but he may mean AT capability vs the panther instead of AT in general.
While I agree that because of deflection damage the penetration buff would make it less prominent, buffing RoF would risk overbuffing the tank. The ISU can already 2 shot 640s and 3 shot panthers with mark and it doesn't need to be done faster. While in all honesty I've had fantastic success against the elephant with the ISU, probably due to the deflection damage, penetration would benefit more against super heavies while keeping TTK vs 640s-960s roughly the same. I don't really think the ISU needs a buff, I'd just like the ability it has to be more useful or changed.