I would imagine that the HP525EFI is destined for the boneyard in the not to distant future. Being a 502 based engine pretty much tells it all. GM has proved that the HP3 is a contender. Mercury is already using the 496 and will expand off GM's HP3. It's just a natural progression.
You have to ask, "Where are the 502 blocks being cast that are used in the HP525EFI?" Is it a Dart or Merlin? Should be at that price!
Personally, you can build a much better engine with aftermarket parts and for less money. I have done it and didn't use any GM parts. Not knocking the Mercury product but $46,000 for a performance motor and drive combo is pretty steep.