You don't need to crack the block open in order to pull the pistons. You do however need a Subaru specific tool to remove the pins. (But I think some people have done it without that tool)
As for the compression test, all you do is pour some oil into the sparkplug hole, and test compression. If it's more than a dry test, then most likely it is rings.
(I do believe that you have to crack the block in order to change the rods though.)