I was recently bitten by the modding bug and called my insurer (Axa) for a quote:
"Sorry, we don't allow modifications".
I would consider my IS300 to be fairly lightly modded (Exhaust cat back, K&N induction, Eibach's, anti roll bars soon) and am personally a good risk (42yrs, low risk job, SDP only, car garaged, 8,000 miles/yr, only driver, 1 x SP30, 10 yrs NCB etc).
Going through confused.com and moneysupermarket looking for quotes, only one quote was returned, at over £900. My previous Axa policy cost just £408 for the standard IS unmodded.
I tried several more of the insurers that confused.com and moneysupermarket don't list, but all either refused or gave hugely inflated quotes.
The best I managed was £707 through Adrian Flux, works out at 73% more expensive than the Axa policy.
I hate to think what some of the younger drivers with the full works are paying.....