I've been an insurance broker for the last 15+ years and the subject of 'modding' cars and insurance is always an interesting one. To say there are grey areas is a massive understatement and the insurance industry as a whole really hasn't adjusted or cottoned on as much as I would have hoped after all this time.
All of these rumours of "you can go up to 20% extra power without having to tell your insurers" or "you can go up to 10% more bhp for every extra £50 they charge you" really aren't accurate and you can't just assume that they are gospel.
Generally in a nutshell 95% of all insurance companies will not insure a vehicle that has been altered from its standard specification, apart from optional extras from the dealer such as optional sat-nav, leather, sunroof, upgraded stereo etc.
Whether a decent modification or not most insurers tar everyone with the same brush and treat someone who has had an ECU remap & DPF bypass just the same as they do when some chav raids the Ripspeed catalogue and fits Scooby bonnet scoops and neons to his Vauxhall Corsa
and it is difficult to be taken seriously.
Also you have to appreciate that 99% of staff at your average insurance company haven't got a clue about cars and their respective modifications and they won't know a remap from a rollcage.
Too many insurance companies see vehicle modding as a licence to print money and will regularly use it as a great excuse to take more money from you claiming that you will instantly become much higher risk to them as soon as you alter your vehicle. Of course though, how are they to know if the changes made to the vehicle are going to be carried out by an experienced mechanic or by some bloke on his driveway?
Some brokers such a Ady Flux have special agreement in place with their panel of insurers where they have pre-agreed certain types of mods with the insurers beforehand, which makes the process a lot easier for their customers. Just bear in mind that only a handful of insurers have done this though and so don't assume that all insurers should tolerate mods and will allow certain bhp increases etc.
The best thing to do is to run any proposed changes to your car by your insurers first to see what they have to say before you commit yourself financially. Regardless of warranties, any changes to your car's specification need to be referred to your insurers for them to consider - FACT.
Whether you consider the modification to be a performance enhancing mod or just something that improves reliability is not important. A mod is a mod at the end of the day and any insurer is within its rights to decline cover.
None standard wheels for example could easily make your vehicle stand out more and could make it more attractive to a potential thief. They could also drastically affect the handling/braking of the car, which could lead to an accident.
Remaps cannot be detected by insurers unless they decide to pay to put your car on a rolling road following an accident - which they won't do. Rolling road figures are rarely accurate too. However, remaps should always be referred to insurers just the same as any other modification.
Also, another point worth mentioning is that people often say that if you modify your car, don't tell your insurance company and then have an accident, that your cover will automatically be voided. This isn't strictly true at all. In the event of any accident an insurer would have to prove that the modification you had made to the car was in some way the cause of or be influential to the accident for them to be able to legally void cover.
Finally, if you don't disclose mods to an insurer and have a claim you can only expect to be paid out on a car of totally standard specification.
Of course, by not disclosing anything you take your life in your own hands though and is it really worth it?