I am installing radiant tubes in the floor joist pockets that I have added on my first and second floors.

I've been told that I need to use an Aluminum Heat Transfer Plates with piping for maximum support and heating efficiency. I've also been told that I just need to connect the ducts to the underside of the subfloor by any means and then insulate the top two to three inches of the cavity to make sure the warm air there moves up through the floor and doesn't fill the room below cavities and ceilings, waste heat.

What are the best practices? How much heating efficiency do I gain or lose anyway?

It's a question of efficiency

Boards with tiny air gaps and insulation below will transfer heat to the floor most efficiently.

A board without insulation will transfer heat above and below.

Tubes with air gaps and good insulation will transfer more heat to the floor than uninsulated boards. Insulated panels transfer more heat to the floor than pipes that have air gaps and then insulate.

Other Aluminum Products
https://www.heat-transfer-plate.com/
https://hw-matels.com/
http://aluroofingsheet.com/