It all depends how switched on and competent the stealer is.
And whilst RedRobin is correct that the stealer tecchy might know the car is faster than normal, this doesn't always ring true - dealers have always been adamant that my GTI was remaped (before I actually did remap it), and the same for an Astra I used to own - all based on how well the car went!
At the end of the day, it is very easy for them to check the ECU flash count - and if there is any doubt, the stealer simply hooks up your car to Germany, and will not only interrogate the actual flash count, but also weather the software is genuine OEM. They may then try reflashing with their latest OEM software, and may then toast your ECU.
You really do need to be honest with your stealer, if nobody else on this issue.