I live in LA and the local Dinan dealer laughed when I asked about "off road only" in California. He said its a damn software mod and swing by and he would do it. Got stage 2 in 45 min and wasn't that impressed immediately but about the second half of day two the world changed. Just got an exhaust so looking forward to stage 3.
No issues so far with my BMW dealer and warranty. Had several small car issues and didn't say a word.
I doubt it hurts your car unless you redline to death on a track etc.