The activation stat (in this case, all attack) increases in % (from 22% to 40%) when you have all the ISO-8 awakened/ascended.

The other stats increase in % when all the equipped ISO-8 is the same rank. Example: When you've upgraded all equipped ISO-8 from 3 star to 4 star, the stat % will increase slightly. It will increase again from 4 star to 5 star, and so on. It will only increase when all the equipped ISO-8 is of the same rank.

And no, when you upgrade the equipped ISO-8s, you don't need to reroll the set bonus. Caution: Upgrading ISO-8s will change the stat type (i.e. Red ISO-8 could change from all attack to energy attack and vice versa). It's safer to upgrade ISO-8s in your inventory, then equip, just like cards.