The plug would be fine in Open, Limited, or Limited 10, but it would not be legal in Production. See Rule 22.5 here:http://uspsa.org/rules/2009_Production_Rules_Revisions.pdf "External plugs which alter the grip profile or extend past the bottom of the grip are prohibited."
Guiderod and spring are legal. See Rule 21.6 at the same link.
Guiderod and spring are legal. See Rule 21.6 at the same link.