Consider the worst case scenario , you have an accident, seriously injure or kill somebody. Your car is likely to undergo a thorough investigation. Not only by the insurers but perhaps by the police vehicle inspectors. Even if they ultimately dont spot the mod , you will have a prolonged period of sweating to do.
Insurance companies do not like to pay out any claim let alone a potentially huge claim and will look at every angle to get out of paying. If they find out about any performance enhancing modification then they are likely to void the insurance. You are likely to either face a very large bill or prison.
Its not only about theft.
Do you believe that we are the only people who know about remaps ? OK so maybe a fiat coupe 20VT or sierra Cosworth might stir suspicions more than a Marea Weekend JTD or similar but I am not so sure that there isn't a gadget that can hook up to the ECU and read numbers / dates / versions and at least indicate that something has altered from standard Fiat issue.
With these remaps also involve uprated components such as clutch. If you end up with an uprated clutch, this could lead to questions being asked.
I wont go on I just caution anyone not declaring mods to insurance companies unless your think it is worth the risk for "a bit extra speed".
Some insurance companies are mod friendly, others will not even insure you if you mention the word modification. Some dont charge more for cosmetic mods others charge by the % over standard BHP in bands. I am speaking from my Fiat Coupe 20VT days.
Just my two pennies worth.