What resistance are you generally running? I find that even around ~0.4-0.6 my tube mech really suffers past about 3.9V. I have a copper/silver mech with excellent performance, but still the effective battery range is pretty small before things get noticeable. I have to assume lower builds would just exacerbate things.
I've recently moved to a dual parallel mech, and while I haven't done any testing for hard numbers yet, I do find that I can get a little closer to nominal voltage before the vape really starts to suffer.
I grabbed a dual battery mod for the battery life, but I've been super pleased with the performance in general. Something to consider if the extra size/weight isn't an issue.