If you're car is modified in any way you MUST tell your insurer. Insurance companies and underwriters themselves are wising up to the opportunities available for modifying cars to give substantial increases in power and torque.
It's not so much that they don't want to insure you, but please do understand that insurance companies are businesses. Like any business, they operate to make a profit. If they can avoid meeting a claim they will do so. Non disclosure is not defensible by pleading ignorance.
You'll find underwriters more accomodating if you can provide evidence of the
mods carried out, by whom they were carried out and the resultant increase in power delivery.
I drive a Peugeot 406 with the 2.2 16v diesel engine. It's been lifted (by means of an ECU re-map) from 136bhp to a healthy 192bhp, with about 330lbft of torque on offfer - yep, that's right 330 and, no, I have not confused this with Nm. As such, it goes like a bloody missile. Even punchy 2.5 to 3.0 litre V6 petrols can feel lame by comparison. It's the torque that you're driving, yet we all talk power! Never done an official 0-62 test, but an unofficial one gives about 7.8 seconds! The best bit is the midrange grunt - 50 to 70, about 3 seconds IN FOURTH!!
Modern turbo-diesel units and superbly receptive to ECU tuning. Please avoid the dreadful DIY boxes - they're likely to destroy the fuel pump.
My insurer is happy to cover the car, obviously for a slightly higher premium, but it's nice to relax and know that I have played fair and safe by all parties.