Hi, I have an insurance license in 47 states and have been in the industry ~8 years on the personal insurance side (home & auto primarily).
1) Most of the time when you hear of people seemingly inexplicably being cancelled for reasons like "overdue for an accident" or underwriters digging hard enough to find a performance modification, it's usually because the company has decided it's over exposed in your particular market and they're using any excuse to thin their book of business in a particular area or with a particular segment of customer.
So Geico's actuaries decide they've taken a bath on one too many collector/limited production car.....well, time to see what they can get off the books, and they go fishing.
2) The Hagerty / Grundy approach will get you the most effective coverage if you are ok with the mileage limitations. While you can't just arbitrarily invent an agreed value for your car...(e.g. I couldn't insure my 1992 with 243,000 miles on it for $100,000 just because I was willing to pay the premium), if you can reasonably defend the value to replace your car with like condition/mileage along with mods, then there shouldn't be a problem.
Some other companies will do a "stated amount"...but that's not the same as "agreed value". Careful with that terminology.
3) The truth is there is no such thing as blanket insurance advice regarding "which company". The reason there are so many companies is that each of their underwriting, pricing, and marketing practices are different and each are strongest with some specific subset of customers. Safe Auto will sell you a hella-cheap minimum limit policy because you have no money, but I don't even know if they would offer a $half million limit. Progressive has a price for nearly anyone, "Is your car on fire outside right now?? No? Ok, here's a rate". Chubb probably won't even answer the phone if you're talking less than half a million liability, but if you need to insure your $10m pseudo-castle they're your guys.
Which company is best for you for "normal" insurance (i.e. non-collector-car) will depend entirely on your credit score, driving record, location, and household composition (e.g. kids of driving age or not).
4) Generalizing some good insurance guidelines (not rules):
Get as much liability as the company offers, this is your CYA coverage against lawsuits
A higher collision deductible can save money on premium and only comes into play if an accident is your fault or shared-fault.
A lower comprehensive deductible may not cost much more to carry and can help with hail/glass/fire/animal-strike
Some companies may offer "full glass" coverage in some states/locales. This waives the comp deducible (or uses a much lower deductible) for chips/dings/replacement.
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Almost every company has a "racing" exclusion these days that they apply to policies that excludes track use. There are a handful that would cover damage at "school" events (no competition), you should ask your agent....without admitting to anything obviously...if your policy has such an exclusion. However there are companies that will offer HPDE coverage by the day with a supplemental policy.