Yeah I used the 105mm on the ARL but switched to the 90mm on the AMX M4 45. The pen really helps at tier 7, whereas at tier 6 it was just overkill pen.
I've found the tank to be a huge step down from the ARL, but it isn't underpowered. It shares a lot of features of the Tiger I. Tiger I can brawl a bit better and AMX is faster and can do medium tank style hull down harassment better.
My advice to the OP is to stop classifying tanks into neat little boxes. Just because this tank doesn't fit what your label of a heavy should be doesn't make it a bad tank. If this tank doesn't have enough armor for you, then you better just quit the line right here and now.
Edit: Basically, trying to play the AMX like a heavy will work about as well as playing the Valentine II as a Light.
Heavies are supposed to have armor though, that's why they are called heavies. This tank unless put on a hill with the enemy in a valley cannot bounce a shot. I've been doing better but only because I've installed a gun rammer and vents plus rations to buff the reload of the 90 mm