1) Because when you're forcing the mixture in, the relative compression of the engine goes through the roof.
2) All depends on the engine really, if you're running in the 8.?:1 you should be able to get away without dropping your compression ratio (Or just a little if you want). If your compression is a fair bit higher than that to start with you'd have to think about dropping it.
3) Pre-Ignition and things breaking, generally not too much fun. There's only so much you can tune/avoid.