just bought an st170 and spend hours phoning round every company known to man. Had one decent quote from a-plan insurance (08459711234) and then went on www.confused.com . i entered all the details bar the body kit/ modifications and got a quote. i then phoned up the company direct with the reference number given told them about the bumpers and still got a cheap quote. tell them the answers to the questions they ask only (but be carefull)
I am now paying less than £400 with 6 points on the st170 with custom built front and back bumpers with a 6" exhaust. Worth a try.
let me know how you go on!