1 - Already do that. Closeby debris colliders have detailed colliders but further debris colliders have simple box colliders.
2 - Destruction already doesn't appear for houses not in view. My previous culling system just used portal checks. My new culling system uses both portal checks and camera view culling. That means that buildings won't destroy visually if you aren't facing them. I wonder if people would notice.
3 - Roof tiles debris is particle system based now. Animations would be the ultimate performance save, but you are right, it would look worse
Ill keep it in mind. Ill have to test it to see if its truly a performance save.