You just agreed with what Grommit and I said after ignoring it. No, there doesn't have to be this:
there has to be some BB/100 out of the SB that we should be able to achieve.
You said you filtered 3-6 handed. How much 3 handed do you play? How good are you opponents at using position. When playing 6 handed, how many pots are limped to you... all this to ask about game conditions.
exactly 6 handed -- SB winning at -9.76BB/100 in ~15K hands
exactly 3 handed -- SB winning at +0.90BB/100 in ~4K hands
So, want a number? Shoot for winning in the SB.
my 200k hand database shows you can, QED. Seriously, I'm over 300k hands in this DB and my 6 handed SB sample is still a lolworthy 15K hands. I've got great games mixed in with tough Stars 5/T hands. I wouldn't draw any conclusions from the WR. Working on my game in the last 3-4 years, I'm a much stronger player in the blinds. I'm lost a lot less often, and I make better decisions. That should be things you look for.
I'd guess anything around -10BB/100 would be fine in the SB. Maybe 15 would be fine. Since you need a vast sample size to use win rate stats, even if the games were relatively the same day to day, I'd recommend other tools for looking at leaks. You lose 25-30BB/100 and I'd think you're probably over defending. For a student, I'd look at cc% vs. 3!% and look for leaks that way.