Tweaking the software in autopilot could cause it to kill you and anyone else on the road. For example, you might accidentally make it not return control to the driver when it can no longer navigate or cause it to accelerate out of control. It isn't clear to me that increasing horsepower or changing the wheels is as significant. There are a bunch of common modifications that are outright illegal like high/low-riding changes, emissions, lighting, etc. though it is on a state by state basis in the US.

http://blogs.findlaw.com/law_and_life/2012/07/the-top-5-ille...