Make sure you get quotes from Amica and Allied. I can't remember the details, but I spoke to an underwriter at Amica about my mods; I wanted coverage for $5k in aftermarket parts. I'm covered. I also mentioned my track events. No coverage, but no increase in rates. The personal lines guy in my office can shop you around to 5-10 carriers if you like (limited depending on your state, CA, NV, AZ ok). He's a broker for joe six-pack all the way up to a couple billionaires. PM me for contact info.
We are dealing with a guy that got T-boned by a kid on a motorcycle doing 70mph in a 35 zone. Somehow the kid lived and by week 3 the bills were at $750k (don't ask me how). The driver had pulled into the motorcyclist's lane as he was exiting a driveway, so there is partial fault. If you think $500k is a good coverage limit, I'd disagree, especially here in CA.
Me: independent commercial insurance agent.
I pay via Amica:
$1650/yr $1M comp/coll coverage
t-boned in 2009 ($27k damage) with Amica.