what kind of equipment are you working with?
a protank with kanger built heads & original coil and wick are very limited, i'd agree. if rebuilt properly, 8 to 10 watts shouldn't be out of the question as far as performance without consequence is concerned (unless it's simply above your personal limit). though, one could certainly make a protank sing like a canary at 6 watts with the right build. even the stock heads sometimes perform decently in the lower powered range.
with a carto tank, 12 watts that some people mention, i think is severely pushing it, personally. maybe i just haven't tried their preferred brand or spec'd carto too. the ones i've tried so far really produce well at 6 watts or above.
then there's the iclear 16B for which i personally feel doesn't really sing until about 8 watts or so, but the wife rather enjoyed closer to 6.5 or 7.
if i'm rebuilding with 28 gauge kanthal and micro coils, i tend toward the upper limit of those ranges i posted. with 30 gauge, i tend closer to the 10 to 12 watt range with a tight coil. the heat just isn't there like the thinner gauges IMO. though, it tends to perform similarly and last a lot longer between builds.
maybe those specifics might clear some things up. maybe not. dunno...