re: If Auburn finishes 7-6, would that make them mediocre?

I think Auburn is a decent QB away from being a good team.

As the OP stated, with the exception of the LSU game, Auburn was at least competitive in every other game. Lots of close losses.

The defense actually saw very little change from 2014 to 2015 in terms of yards and scoring.

However, considering this year's defense had an offense rated #95... and last year's defense had an offense that was #17 to complement them, I'd say Auburn's defense was probably "better" this season despite it not showing in the statistics. Had Auburn had a Top 20 offense this season like last season, my guess is the improvement on defense would have been more evident in the rankings.

Unless some folks declare early for the draft unexpectedly, it appears Auburn will have a good bit of their defense back next season. If they can continue to improve despite a change at DC, I think Auburn could have an "average" defense which would be a great improvement from the past 3-4 years.

So it appears next season will boil down to the QB position. If Auburn's offense can improve from #95 up to even Top 40, I think Auburn could see improvement up to an 8-4 or 9-3 type of season.

In order for Auburn to compete for championships with the coaching staff though, they need a Top 20 offense.