When you compare the Panther A/G/F to all their peers in the tech tree, they don’t compare well. To make some comparisons:
Tiger H1, firepower: The Panther has better pen and velocity, though this benefit is marginal considering WT is mostly CQC, and the extra pen doesn’t usually make a massive difference, e.g. it’s possible to pen a jumbo UFP, but in practice unlikely. On the other hand, the Tiger has significantly better HE filler and post-pen damage, in comparison to the Panther which has little and is has somewhat of a slow reload in relation to the damage it can do.
Armour: The Tiger has a better armour layout, with significantly better side armour, and double the turret armour, meaning the Tiger is far better in hull-down positions. While the Panther has a better UFP, the Tiger can simple angle to get similar protection. The Panther only really has better armour against frontal threats that you haven’t seen, hence haven’t angled against, which isn’t that common, while the Tiger is far more survivable in any other situation.
Mobility: The Panther isn’t significantly faster than the Tiger. On the other hand, the Tiger is significantly more agile, with significantly faster track traverse and reverse speed. This is a massive advantage as it takes simply ages for the Panther to reposition and means you can use “peek-a-boo” gameplay.
Gun Handling: The Panther has better turret traverse, and is more stable on the move, though this is a minor advantage and only applicable on flat ground.
Other: The Panther gets a roof MG.
Overall, these two are have advantages and disadvantages, but are similar in overall strength, so they should be the same BR at 5.7. The Panther has some advantages in some areas but the Tiger has big advantages in agility and survivability.
Tiger E: Same as the Tiger except it has a roof MG and lacks the massive cupola weakspot which both the Panther and Tiger h1 have, which means it’s basically immune to small-calibre guns from the front. Clearly it should be higher BR than both the Tiger H1 and the Panther which has big weakspots that you can’t get rid of.
Panther D: The later Panthers get a big advantage in turret traverse as well as some other bells and whistles like slightly better side armour, better zoom and roof MGs. However, the benefits are partially counteracted by a big decrease in mobility, which hurts even more as the later Panthers are not effective frontally at the BR they play at - so they need to flank. This decrease in mobility hurts their flanking potential and means they often can’t get into favourable positions before US light tanks. Meanwhile, the Panther D and VK can be effective at even 6.7 by getting into favourable spots early and using its high velocity gun which can pen the side armour of most tanks. The later Panthers should be higher BR because they get one big advantage and a few smaller advantages in exchange for one major disadvantage, but probably not 2 BRs.
PZIV/70: The PZIV/70 is a casemate which is a big disadvantage, but this is made up for by having a significantly better armour profile - essentially no frontal weakspots, along with a far smaller profile which makes it less likely to be hit in the first place. It should be the same BR as the Panthers.
So in conclusion, Panthers feel more like 5.7 tanks than 6.0 tanks. You could make an argument for the Panther F to be 6.0 since it has a rangefinder, better zoom and better side armour, but it seems like the G and especially A make more sense at 5.7