It's much easier to react to dives in the SU85 than it is the JPIV. Both are slugs but the JPIV is a slug on heroin in reaction ability.
Mobility matters quite a lot here, not saying ROF isn't important, but can the JPIV have at least one significant stat that's superior to allied TDs?
JPIV is a great unit but it's not a hard counter to allied TDs, it's up to the superior player in these types of engagements.
It might have a leg up in certain situations (lanes) but it also has a leg down in others.
It's mediocre against Comets, Churchills, IS2, ISU, Pershing. Comets get spammed as much as Panthers.