The maximum increase in performance with a particular stat will be in moving from:
minimal extensions, T1 main mod, no boost mods
to:
max extensions, T4 main mod, max number of T4 boost mods
For example, using the same bot and gun class, considering DPS alone, how does:
player with only the extensions required to fit the equipment, and T1 guns only
compare to:
player with maxed DPS extensions, T4 guns and a full rack of T4 tuners?
I suggest AO should set specific goals for how widely a given stat can differ between these two extremes. The numbers Alexander posted on demobs show a 108% increase without even maxing out on the tech level. Decide whether that's acceptable or whether you'd rather it be closer to a (e.g.) 50% maximum possible increase. This will help put boundaries on extremely focused fits and help ensure that a variety of mixed-focus fits are reasonable options.
Caveat: It's only for bots of similar class and specialty. Obviously EW bots should have meaningful EW advantages over mining bots. And this would help most in balancing specific stats against themselves, which isn't the whole of balancing. But I do think it would still go a long way in preventing other balance points from getting too out of hand.