Flash 4 syntax in Flash 5 and 6 SWF

reading kendude's game development thread and mmdev's respones reminded me of the efficiency issues with Flash 5 ActionScript engine.

I seem to recall that Flash 5 used the Flash 4 ActionScript engine but allowed support for EMCA dot syntax. However, I believe you get better performance if you use Flash 4 ActionScript syntax with Flash 5 formated SWF:

use tellTarget
use slash syntax for target paths
use get/setProperty functions
use Flash 4 string and math functions instead of string object and math object, or use Branden Hall's string, and math prototypes for Flash 5 instead of built-in.

First of all, is this a correct understanding of Flash 5 actionscript performance?

Second is it generally a good idea to just go with Flash 4 syntax even if you are coding a for Flash 5 mobile platform such as Clie, Motorolla phone or Communicator devices?

Third, is there any performance loss when using Flash 4 syntax in PPC Flash 6 player?

I did not start writing with AS until I got Flash 5, so I am not familiar with the older syntax. From what I do know, the Flash 4 syntax would be better since I believe that is what Flash Lite is all about. I usually write my stuff for MX, but then I have to go back and change it for the mobile devices. I have only written two games for the phone. One is tile based, the other is a parallax scroller. I would think that Flash 4 syntax is probably the most efficient in any player even though I can't back that up with hard evidence.