If you take off weighing above the MTOW and have an incident that involves an insurance claim, the insurance assessor will calculate your total AUW at take-off and if an ounce (gram) over your MTOW for that flight, they will not pay out, and you may also find the CAA chasing you for flying illegally.
Families of any fatalities may also sue you/your estate for the death of their loved one and your gross negligence! Is it worth it?