I got my 05 STi about 3 months ago, I'm 22 and I got full comprehensive with SGIO for $980ish or there abouts (sub $1000). They didn't offer me the cheapest quote, but they'll beat any price by 10%

RAC suprisingly had the cheapest but any mods I did (even a different diff back) they wanted to know about, know a value and any power increase and some other things. It was a bit unreasonable. So I got SGIO to beat their price by 10% and SGIO said to me over the phone they don't care about mods as long as they are road legal and the car is road worthy they will insure up to the agreed value. They also put in free window replacement to match RAC

agreed value is $30,000