Like Dai, I phoned up my insurer (Admiral for this year and last) and asked about adding a stainless exhaust to the Coupe, because the old exhaust was rotten and a stainless unit will not rust, and will actually cost a lot less than an equivalent system from Peugeot (£420 from Pug for mild steel, £360 from Powerflow for stainless)
They told me straight away what it would do to the cost of my policy, and I decided that it was still worth it. I booked the appointment for the fitting, then called Admiral straight away and asked them to make that change dated from the day of my appointment. On the day of my appointment, I had a letter through my door from Admiral with a revised motor insurance policy document showing the modification as "exhaust/manifold - 3rd party". I take this to mean that I could have replaced the manifold with a stainless unit as well and this is still covered - but there's probably not much to gain by replacing the exhaust manifold with a stainless system.
Same when I got my wheels - before I bought them, I phoned them up and asked them what it would do to my policy. They told me, and so I bought them, and when they were delivered, I phoned them up and told them when they were going on to the car. No problems.
I haven't had any issue with Admiral; they have been good to me so far.
The fuckers that pissed me about with the old 406 saloon were Direct Line. Named and shamed
I've no heated seats, no sat-nav, no rear blind, no 'folders'..... 
My spec and his engine eh?? 
