It may or may not be a horsepower issue, but a combination of factors
involved with certain models. Each model has characteristics that make up
it's accident survivability profile and an accident history each it's own.
Whether high stall speeds, high or low Vmc or Vyse speeds, thin fast wings,
low stall speeds, fat high lift wings, rudder size, cabin size, cabin
structure, engine size, turbochargeing, prop rotation, pressurization,
complex fuel systems, or center of gravity issues, every twin is different.

When we were shopping for insurance for our Aztec we were told that
insurance companies look favorably on Aztecs because they have "enough"
horsepower to provide adequate single engine performance, but not "too much"
power that single engine control problems overwhelm the pilot. Our agent
told us that the Aztec premium would be less than a Apache of equal hull
value given our times and ratings. He also went through the list of
companies willing to insure "transition" pilots (moving from HP Singles to
Twins) and noted which company would insure which models with what pilot
experience levels and for what size pile of cash.

YMMV, but it sounds like you're on the right track and that the agent at the
AOPA insurance agency knows what he's talking about.

Good luck,

Jim