I often disconnect my battery and when reconnecting back, the SMG dosen't slur etc. Do get the thud thing but it only happens if I had it sitting around for a couple of weeks. For me the SMG has always been a little mechanical when I let her sit abit. After a few blasts I don't really notice anything unusual from no battery power except the crisp SMG shifts.
My input probably isn't much help but I've always found the SMG to be a bit clunky at low speeds.