Well, I guess I have rather radically different opinion on German tanks than the community here. I am a total amateur, so whatever I write please don't take as holy truths. May I also remind that I don't claim that panther was the pinnacle of tank design. It obviously had issues. Anyways, let's do this..
They used them as other countries without their own tank fleet - until the spare parts were available. I completely agree that they could have been complicated to use, don't get me wrong. They weren't designed because someone wanted to overengineer but you can't avoid the fact that no attempts were made to improve the tank.
Every MBT today uses panthers armour philosophy, all or nothing, focus on frontal protection with addition of 60deg frontal arc. You enhance panthers turret side armour and add heavy side skirts to the frontal third of the hull sides and you have western armour arrangement of the 70-80 ies. This was a deliberate move. From 1943 on they had the only army that operated a mainstream medium tank with actually functioning armour, excellent armament and decent mobility. You can call it all mediocre all you will but criticism of a panther is unavoidably conditioned by its era in which everything else was worse! If you dislike Schachtellaufwerk you have to confront the fact that German tanks featured more armour, better relative mobility and better weapon platforms. What was a better alternative? Even a fairly good IS-2 only a tad better armour than tiger 1 whilst dispensing with a load of features. Turning radius, weapon stability, barrel depression, ammo storage, single piece ammo, reload speed are all things that matter in tank combat but maybe not so much for someone that doesn't expect a tank to have time to spend its ammo.
I seriously doubt that any professional would consider D-25T a better weapon than either KwK42 or KwK43. D10 was afaik considered quite better but even that one could not match the long 8.8. I understand the big soviet calibres rather a necessity born out of resignation of the ammunition and cannon designers. Good HE shell was probably second requirement. People today spin in circles thinking that cracks in German armour are an indication of a good anti-tank weapon.
Most late and immediate post war requirements evolved around Tiger B's 8.8 kwk and its armour. Most modern MBTs likewise focus on good mobility and have primarily anti-armour armament. You will perhaps notice that nobody managed to get the German performance out of their guns. 17pdr is the closest but at a cost of a heavier shell. Everyone else had to choose between custom ammo or bigger calibers.
But let's not forget. French immediate post war medium and heavy tank development is all German designs (besides that fetish for scillating turrents). Lorraine 40t, Cannon D'Assault Lorraine, AMX-50.
Centurion had panther's hard specifications. Two years younger tank with no advantage over a panther and which cut the edges at mobility and suspension (unless you consider horstman any credible tank suspension). If Germany wasn't collapsing it would compete with panthers with up to 120 mm armour and 8.8cm cannons. But all tanks suddenly look better after war because Germans designed no more, coincidence?
Translate the weight difference into reliability, add logistics and you are not far off.
As the heretic said, your stats and calculations are wrong. I got to 0.31 W/ccm for a HL230 and 0.26 for V-2 whereas I took 370 instead of 340kW. But more important is that the engine is also shorter. This means shorter hull, less metal for sides and shorter track lengts, which is crucial for any decent turning radiuses. Usually neglected is the fact that turning radiuses of shermans and IS-2 were giant and you can forget about any credible tactical maneuvering.
No? Well, what was equal then? Neither T-44, Cent nor IS-3 were battle read by the end of the war and panthers were rolling around for 2 years by then. Anything else is pure conjecture.