What ohm heads are you using? The voltage drop is going to be more noticeable with the higher ohm heads since you're naturally going to need more voltage. I've not once noticed a drop in vape quality using 1.8 to 2.4 ohm heads vaping comfortably at 6.5 watts on protanks and 7.6 on evods. Back when I used Novas, I couldn't even notice vape quality drops at 8 watts, nor at 10 watts on DCTs until about 20% at 10 watts (18350 is noticeable for me past 8 watts on 2.5 ohm heads/cartos between 8 and 10 watts, but under 8, I don't notice a thing). I've had my evic for about 6 months now and I've been vaping for almost a year and I noticed quality drops on spinners/twists/kGo VVs and of course the unregulated mini/eGo batts, which I haven't run into with the evic unless I'm pushing high voltage. Regardless of which 18650 battery I've used in my evic (or even Vamo for that matter) I don't notice the drops unless I'm trying to push more than 4.3 volts (unless I double stack 18350s in my Vamo, which doing so in your evic will completely destroy your control head). The only real thing I can suggest is drop down to 1.8 ohm heads and try to keep voltage below 4.2 or so. If the stock coils aren't cutting it for you, I'd suggest trying to build a 1.5 ohm or so micro coil and wick it with cotton for your Protanks, and if you're still having problems, well, not much we can do for you then.
Could be defective batteries, could be a defective evic, or it could just be you.