var from your sample is not an object in ObjectScript. It is an object reference.

So in first case you pass object reference by value.
In second case you pass object reference by reference. That is reference to object reference.

I don't think you should expect noticeably better performance

Look in "Developer Preview". It's not released yet

If you can select from this stored procedure then you can try to use this select in INSERT into.

Does the following work?

SELECT * Custom_MENS_Other.samplesp('2021-11-02','2021-11-04','H001')

See Table-Valued functions section:
https://docs.intersystems.com/ens201815/csp/docbook/Doc.View.cls?KEY=RSQ...

And INSERT with SELECT:
https://docs.intersystems.com/ens201815/csp/docbook/DocBook.UI.Page.cls?...