I agree with some of what you're saying Paradroid. But what happens if you crash your car which you've fitted the TTE bumper too, Lexus have a look at it and notice the bumper, then do a little search and manage to find a pic on the Internet of your car without a TTE bumper on, then refuse to pay out ?
It might sound a bit far-fetched, but I'd rather be on the safe side. Especially since mod-friendly insurers probably won't be bothered or increase your premiums for that.
As you say, performance-related parts are going to be of more interest to the insurers, but that doesn't mean you shouldn't declare the other bits.