I never have fast arty on.
Merkava's went to the heavens in one hit.
So did any vehicle it hit at other parts of the battle. Be it tanks, APCs. Yes, TOS-1 series.
It has less effect than CM artillery in the sense that it destroys only what it hits (hex), where CM artillery doesn't care about that, it will destroy and damage tanks not only in the same hex but all the neightbouring+hexes in a radius of 2+1 hexes. So yes CM arti is far more powerful in that you have a better chance of hitting enemy.
But yeah, I stopped using them both PBEM and against the AI. They feel very unfair.