It's hard to compare GAU-8 to FFAR. Two different animals. Even if GAU-8 AP round won't penetrate as thick armor as your beloved Mk.5 does - remember that in case of GAU-8 the amount of rounds that hit the target counts (remember it fires 30 rounds per second!), so the A-10 can get effective mobility kill even on heavier tanks if only it hits the engine bay area, tracks, sensor suite etc.
DCS dammage model on AI controlled vehicles isn't sophisticated. It's poor. I would argue it's worse than OFP's dammage model. Don't overestimate it. ED just had to model these mobility kills somehow in the sim so it's will be "playable". It seems it's not the case with FFARs though. And I find the hydras more than sufficient to kill BMPs, BTRs, BRDMs, MT-LBs and various lighter targets with a single hit. I don't think they are too weak.
Simple as that.