Claudius,
you change the exhaust on an Evo
The original poster specified it was the ECU, not exhaust. Don't worry, alcohol affects my ability to read as well.
the insurance company want an extra GBP 556 or so and you think it would be stealing FROM THEM not telling them you have it?
How about this ... you change your ECU thus making the car faster. Statistics say that this also makes the car less safe. If you don't tell the insurance company, you are, effectively driving without insurance. Now, you crash into another car, your insurance company finds out you're a liar and decides not to pay up. Even worse than that, they don't pay the driver you crashed into either, and that driver has to go through all kinds of hassle making an uninsured loss claim. They might still lose their no-claims bonus, which might cost them several hundred pounds over the next couple of years, even though the accident wasn't their fault. Of course, once they find out how clever you'd been by saving a couple of hundred quid by lying to your insurance company, they'd probably say ... Gee, this guy's so clever and devious I'll just forgive him for everything he's done. In your case, he'd probably ask your Daddy nicely for some compensation and all would be sorted.
that takes fückin ages to rev up.
We've already established in a previous thread the reason you think it takes ages to rev. up is because you don't know how to change gear

(I know ... when I was 17 and just learning to drive I also found it hard to negotiate the tricky route from 2nd to 3rd gear).
On a serious note ... the reason why we pay so bloody much for insurance (especially in the UK) is because of dishonest and irresponsible tossers who think it's really OK to try to avoid paying just that little bit extra, or even not at all. The system is annoying and we all hate insurance companies for the right reason, but it's not just the insurance companyt that's affected when someone decides it's clever to hide a material fact from them.
The high insurance costs are also down to those annoying no win no fee so-called legal firms who encourage anyone who broke a fingernail in an accident to sue the living daylights out of the insurance companies ... but that's a separate topic.
Ian.