I am not sure in terms of stats but from my own experience Panther can take on a Comet and vice versa, PzIV can take on a Cromwell and vice versa. Firefly is a gimmick tank. I don't use them unless forced. Can't really say a lot but they are very fragile and fire slow. Something with high RoF like JgdPz IV or StuG III should be able to fight it pretty reliable. Provided you can dodge Tulip.
Yes, the Panzer IV can take on the Cromwell, but the outcome is dubious at best. That should simply not be the case for a tank that is noticeably cheaper than the Panzer IV, is more mobile, has better accuracy on the move (actually allowing you to perform flanking manoeuvres without permanently shooting into the ground), and then also has the better gun (which also deals deflection damage, similar to the PIAT).
That wouldn't even be the biggest problem, if British teching wasn't so cheap and fast in comparison, that a Cromwell will certainly come out earlier if both players have a roughly similar fuel income up until that point. Something here doesn't make any sense, and it is certainly not the Panzer IV being too good.
The Comet/Panther match-up isn't quite as drastic. Yes, the Panther has slightly better acceleration (but lower max speed), costs a tiny bit less, has more frontal (but lower rear) armour, more penetration. But in come those small details. The Cromwell reloads a whole second faster, has deflection damage (which means it can deal out damage despite bouncing due to its lower penetration), and again the lower movement penalty - which is why the high speed of the Panther often comes to nothing, because shoot and scoot doesn't work.
Combine that again with the teching cost and time (the time here being somewhat more close, due to the additional research going into the Comet-sidetech with the Brits). So the Panther/Comet battle if much more close than the other one, but still there are some weird discrepancies for a faction that for the entire game is forced into a defensive role, having the battle dictated by the enemy all the time - what was meant as the selling point of the Ostheer originally has now become its greatest curse with the way mobile units are pumped up by the game's mechanics.
And in comes the StuG. Right now it works as somewhat of a stopgap between the Panzer IV and the Panther. But the decision is still between the StuG and the Panzer IV, and that doesn't work - because both were originally meant to complement each other, which is still visible in how they play. But as it stands now, you don't have the resources for that. Which results in you having to decide between either, instead of getting both (in the short run). And that doesn't work well for the StuG, because it of course can't compete on its own against turreted tanks, because it lacks the range of the SU-85 and the Jackson (I know, it costs less. This is just an analysis, not a balance suggestion.).
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So what could be done to lessen these impacts? Well, the most obvious way would be an overhaul of the entire teching system. That way we wouldn't have to rebalance the Sherman, the T-34 and a whole lot of other things to a reworked Panzer IV. First of all, remove that stupid research/building combo. No other faction has to pull back fighting forces from the front AND invest time into research. It is always either or, not and. The other idea to bring up Ostheer infantry up en par would be an overhaul of the reinforcement mechanics. No, not forward retreat point, as some resemblance of asymmetrical balance should be kept. But as of right now, the necessary manpower to sustain a slightly unfavourable position are significantly higher, despite your troops not being as effective. This is mostly a gripe against Tommies and Riflemen, as both are retardedly cheap to reinforce with a decent performance per model. Grens lack that, and it shows. These cost/performance differences need to be looked at, and overhauled accordingly.