I know that as you buff them, they state:

Reduced damage taken / damage multiple targets
Reduced damage taken / aoe hits 5 targets
Reduced damage taken / aoe hits 8 targets

I don't have a parser, but I've solo'ed extractors a few times and the ones with level 4 turrets are consistently much easier than the ones with level 1 turrets. Has anybody tested/parsed this to see the effects of incoming / outgoing dps on turrets at different levels? From a soloing standpoint, I prefer finding extractors that the other team has gone the extra mile and buffed them all to 4...