While I'm sure the Amsoil coolant is excellent quality, and of course I'm an Amsoil dealer and use their oil exclusively in our engine builds, I can't imagine that there's any significant benefit to overall cooling system performance with Amsoil coolant. It's great for cast-iron engine blocks and preventing corrosion, etc - but there's no significant benefit to the Subaru platform..... It's not going to make any real difference in system performance.
GD