i dont declare any mods, never have, never will, and i dont know anyone who does. oddly enough i dont know anyone who's ever had their insurance declined for undeclared mods either. and i know plenty of people who've made claims. coincidence? i think not. the main issue is that your insurance company will very very rarely look at your car, it will never happen if you're TPFT because your car wont be getting fixed. if you're fully comp and you crash and its your fault, the place you send your car for a quote and repairs wont be your insurance company, just an approved repairer, and even if they notice undecleared mods (one in a million chance) they wont say anything because they want to get paid by your insurance company.