@ moen,
Well, some of the features in Platinum are still greyed out because 321studio has not perfected them at their last release - which is 3.2.1. I am sure you are talking about such features as acquiring TDF files from online, or DVD files from online - really not important features to make good backups of your movies. You know that Platinum is really Xpress (but with the ability to remove content from the DVD)... You can tell when you check the content that was copied to your computer, the folder that it all goes to is an Xpress folder... :P
In terms of speed - well, I've burned at all speeds successfully... But to play it on the safe side, I usually burn at 1x, or 2.4x. But like I said, burning at 4x works out just fine for me as well... I have not had the opportunity to try out 8x yet being I have no 8x media...
As for the burner I use, check out my sig.
I am using the same burner as you - Plextor 708A. I moved up from the Plextor 504A. Both have been great drives for me and I have yet to burn a single coaster.
I too have both DVDXCOPY 1.5.2 and Platinum 3.2.1 - obviously purchased as well. You shouldn't have any issues with having them both installed at the same time because they are installed in seperate directories.
-SlickVic78