You'll usually break even or slightly above using HEAT in a tier 5 tank with premium. If you miss a whole bunch, well then you can easily be out 10-20k. If its at range, and it moves, its not easy. The gun is also amazingly accurate considering how large the circle still is when fully aimed. They do excel at brawling and with a 6.6 sec reload, equal tiered tanks are dead fast.
Also the M6 is a good tier 6 tank if you need to make credits. The 90 doesn't require gold use at all, unless you want to pen tier 8 tanks frontally. It also leads to the T29, which is probably one of the best tier 7s in the game.
The panther, panther2, E50. Ugh. You can feel your modules breaking just from tanks looking your direction. You would also think those things are made of kindling and doused in gasoline. The panther 2 will lose a fight to any other tier 8, and while it may be a good sniper, its camo sucks, and once spotted, nothing really bounces off it.