Honestly, you're probably better off asking these questions in the cloud chasing section of the forum. That said, if you're running a mech at .2 ohms and your clouds aren't big enough, the best way to improve them is to work on your inhale and exhale technique.
With the Sigelei, I agree with the poster who said to try wrapping bigger
coils. You can turn it up higher, so you don't need to be super sub ohm, and a bigger coil will get you bigger surface area to vaporize more
juice. With something like a .5 or .75 ohm coil using more wraps, you'll get much better vapor production at higher power levels.
It's not all about super low
coils. I'm running a single coil at 1.4 ohms at 23 watts right now and I can pull a pretty decent cloud out of it when I want one.