I have used a couple of metal barrels, esp. if there isn't a muzzle brake on the end. I built a SU 152 and have a M40 GMC I plan to build. Those are big guns and the rifling in the metal barrel on the M40 looks cool.
if I recall, and Karl or Rob will know, the muzzle break on the Tamiya Pz 6 is grossly over scale.
I do think individual link tracks make all the difference in the world. Whether plastic or metal, kind of depends on what's available. I used the Dragon Magic tracks on my 152, following Karl's toot. Because that gun has return rollers, it was easy to shape them up/ down to look great.
For my T55/ Tiran 5, where the track runs back over the road wheels and then has a long free run, Friuls just couldnt be beat for the realistic hang.
As far as exterior sheet metal, I suggest and this applies to all scale models; work from photos. What's in my mind as "right" isn't often.
For stuff like exterior armor plates, more often it goes missing.