flouris, the number of hits you get really depends on the herb. Some grades vape better than others. The first hit will always be the biggest and most noticeable. No matter what the grade herb, I always get at least 3 hits, but have gotten as many as 7. Not huge hits, but definite vapor.
Also, often people think the stem is cashed, when actually there are several more hits. Have you tried blowing into a light that is shining away from you or into the sunlight? This takes a little practice to get the right light but once you have it down, it's foolproof.
As far as leaving the stem in, I usually do, although there are times I will clear the stem of vapor and put it back in. Your abv should be fairly brown when your stem is cashed, but not dark brown. Fixed temp vapes will not get your abv as brown as a desk top/whip type vape. Often, people will say they can get one or two more hits from their whip style vape from the abv of the log style vapes.
I know the norm is to have your herb dry when vaping with these vapes, but often I'm in a place where the humidity doesn't allow this, no matter how hard I try. Also, some med grade herb that I fell into is pretty moist and gummy. There hasn't been a decline in hits with the moister herb, however. So I'm not sure that is a factor. Grind, however, is and your herb should be the consistency of fresh ground pepper.
In any case, the 13.5 is a small investment and if you think it will make a difference, there's no harm in trying. Just be sure to switch back to the 12v when not vaping.