You are imagining things, or you simply encountered a naturally harder enemy. The first insight threshold is around 15, and even then enemies don't get tougher or get more damage per say. They do get things like special effects on their weapons, new attacks and other VERY visually obvious buffs. Like a flaming effect on their previously normal weapon. Or they start shooting projectiles when they were purely melee before.
Same enemies in same zones will aways same amount of hp and damage. Some enemy types will have different hp and damage in different zones, but enemies in zones do not suddenly hit harder. Unless they gained some VERY obvious visual effects.
High insight allows you to...unlock things that you didn't have access previously. They are purely optional, but very interesting. It's a trade-off. Slightly harder enemies for...well, you'll find out