I like Mk IV and so I like to remind that Firefly was rather specialised vehicle, with strong emphasis on the A/T role and without a decent HE round at first and after all tank wasn't a pure A/T weapon, usually the basic ammo load of a tank had under 50% of A/T rounds. Also at D-Day most Shermans were armed with 75mm gun, just because of its good HE round. Only after first contacts with Pz Vs brought enthusiasm towards 76mm gun to US field commanders. The lesson that one also needs a decent AP performance for tank gun sunk in rapidly.
IMHO Mk IV was a decent tank but in 43 onwards it lacked defensive power and its chassis and suspension couldn't take more weight. Also its suspension could have been better, same can be said also of that of Sherman before HVSS. Late Mk IVs would also have needed a bit more powerful engine.
Juha