I can't explain this but I have an 2002 base Acura RSX (not the -S model) and our insurance company, Encompass, classifies it as a high performance vehicle. It's a damn Civic. I pay around $900/year for that one and expected to get raped when I bought the M3 last month.
Nope, M3, full coverage, $1k deductible, $550/year. The ins agent ran it three times to make sure.
I was going to raise holy hell when I found out not long ago that the RSX was so expensive to insure but we're going to get rid of it and since the rate on the M3 is so good I'm just going to keep my mouth shut.