The ability of the timber-lynx fastener to handle the imparted thrust is a seperate issue from the posts ability to handle the bending moment created by moving the tie down.
Resovling thrust before it reaches the plate will reduce the moment and is in my opinion, always part of good design.
Only an analysis will tell you if the collars are helping with this. If they are near or above the bottom third, it is probable they are not.
The slate should only be considered after someone extremely knowlegdeable looks at your frame and the planned alterations.
