When you quote online or over the phone with SGIO (when I did it anyway) they don't ask what modifications have been done or what the value of them is - they told me they just insure all road worthy/legal cars up to the agreed value.

Most other insurance companies will want to know and the mods you have mentioned will affect the price - With mine, I got a quote from RAC for a stock my05 STi and then got SGIO to beat this. My car is pretty much stock aside from a few bits (has a decent sound system which can add to insurance) anyway, but I didn't put this in the RAC quote as I wasn't actually going to get insurance with them so it didn't matter it was just to get the cheapest quote. I did have to email the quote to SGIO's head office over east before they would approve it (I was completely insured for this period though) as it was about $700 cheaper than their quote. It's your best bet for the cheapest insurance IMO