Actually further to this, I just realized that the way this works causes other minor issues with the Research tree.  I'm guessing this might be a known issue but anyways....

Because there are 2 versions of the Guardian Statue/Idol Improvements (one for Empire and one for Kingdom)... you have <HideInHiergamenon>1</HideInHiergamenon> set for 1 (the Empire version)... so that it only shows up once.  OK great idea but....

The problem is, if you play a Kingdom faction, you don't see this Improvement in the Research tree under the appropriate tech, doh!

That's a bit unfortunate.

My suggestion would be to ignore that flag (in the Research tree) and show the Improvement even if the <HideInHiergamenon>1</HideInHiergamenon> is set.  Because...  like the Resoln shards, and the Guardian Statues... each faction will generally only have 1 version of an Improvement available.  So just show all Improvement available to the faction it in the Research tree regardless of that flag.

Then leave the Hiergamenon behavior as is.

Hope that makes sense.