It would be best to disclose this to the insurance company. The premium should not go up too much. Will be the bets thing to do. Don't want them complaining when the time comes to claim..
@ Dolby , if its only a software upgrade and it happens you are involved in an accident,wont it be possible for investigators to varify if its stock or aftermarket software?
Guy in the office has iirc the second fastest Polo TDi in SA (was the fastest at one stage) - the only thing that disclosing his performance mods did was up the premium because the value of the car was higher. He says it did not go up by some ludicrous amount (YMMV of course)