It requires a lot of work and without mentioning the scripts, although it's not a bad idea, the only problem I find is the performance with which those lights will count. and also to mention that as a result of that people will ask for these same lights in the bumpers, and good I support you in the idea +1
Having sirens would be cool as tuning option, would be annoying when overused. Think about how many newbie Mananas will put one on their car because simply it's the most interesting visual tuning available. Unlocking it should be at least consumes one's soul hard enough so that only those who are really dedicated into obtaining them can have it on their car and making it feels even more special.
If that were to be on the server it should require a high amount of bodypoints or taskpoints even so it won't be overused. Imo 10 taskpoints or 30 body points will be a good price for unblocking such a unique feature. Also yes, +1 for this suggestion.
I was thinking about a hard task such as destroy X police cars, do X time pursuit or something instead of shitload amount of points to unlock them. Well, whichever works, i don't really mind.