Regular Gas in a Premium Engine

I was just reading about the new 2005 Toyota Avalon. It sounds great! But in
the literature I noticed they point out the 280 hp engine uses premium 91 octane
fuel. Although it seems you can use regular 87 octane without harming the
engine, you won't get 280 hp. My question is how much horsepower and torque
would the engine make with 87 octane fuel?

I suppose many engines run fine on 87 octane but only get their maximum
horsepower with premium fuel. So in general - on any such engine - how much
power would be lost when using regular fuel?

Thanks in advance!
Michael