It has happened and it does happen. Take this example: Bike is stolen and it transpires that the cans are fitted. The insurer says that they would not have insured this risk if they had known about the cans and the cans represent an enhanced theft risk. The claim is not paid. Insured loses £7000 bike.
Have never been aware of any other instances or circumstances, but if bike is damaged, many insurers will only pay up to the value of standard parts.
Best to declare all mods.