The issue with mods is; if the insurance company does not know about them, it's not that the company won't pay to replace your mods, it is the fact that by you not disclosing them you have misrepresented information on the application, your contract with the company, as you stated or signed that your car was not modified or customized. The insurance company will use that fact as a reason to deny coverage. Not a huge deal if it just means you pay out of pocket to replace your own car when you roll it into a ditch, a life destroying/family ruining ordeal if you are being sued for $1.5 million after crashing into a minivan and injuring a family of 6.
If the car is not a daily driver and you meet certain criteria (age driving record etc..) then there are companies that will insure whatever you want: coilover/airbag suspension, engine swaps, turbos, superchargers almost anything aside from a pro-street car.
The big trouble comes when you want to commute in your modded car. The only guaranteed option is through the Facility association (the high risk guys). CONS: They will require an appraisal. They are not cheap - the policy will easily cost twice what you would pay for a stock car, if not more. PROS: there are no restrictions on use, mileage, ages, mods, or driving records.
If you are with a middle risk company (ie: Pafco, Jevco, Perth, Echelon, etc...) they will almost always decline a mod of any kind. The philosophy is that mods and claims/convictions = paying big claims. If you are in a middle market it is almost always because of a driving record issue.
Getting a modified car insured through a standard market (not high or mid risk) is tricky. I'll get two different answers from the same insurance company depending on who I speak with. If you have a good record, are over 25 yrs old, and have a few tasteful mods there is a chance that you may get a favourable response.
1998 328ic - Sold