Ok, I had a look at SONAR X3 Reference Guide pages 1466 to 1469 that is a product comparison for the different versions of X3 and compared it to the link Karyn gave us.
 
If you compare Sonar Platinum to X3 Producer then "The Studio Mixing FX Suite" is the Nomad Factory Blue Tubes Bundle plus the Nomad Factory BlueVerb DRV-2080 from X3 Producer.
 
The other suites are all plugins that would have gotten installed with X3 Producer. Same with the ProChannel modules.
 
The X3 Reference Guide also shows the Loopmasters, Sample Magic & X-MiX Content Collection being standard although I don't remember installing these.
 
Same with the instruments. The were all included with X3 Producer except for AD2 (I did the upgrade path from AD1 to AD2 when I got X3).
 
Note that the items in bold are new to Platinum.
 
So as stated in another thread if you have the full install of X3 Producer you would only really need to install Sonar Platinum, the Anderton FX Chain Collection and the SONAR Help and Tutorials.
 
Hope this helps a bit more.

Thanks for this, although I haven't seen this "stated in another thread". If this is definitely correct then it looks like i have nothing to worry about! Shame I wasted my time downloading everything though. 
But...
It is still confusing, and obviously to more than just me! Others advised to install "everything" and to "Install the content in the same place as you did for previous versions and you'll not have duplicates."

Surely this is something that Cakewalk should have clearly informed us of? In fact, at this moment, I really regret spending the money on what appears to be "tweaks". That said, I can't really complain as I haven't installed it yet so I suppose I should reserve judgement until I see what it can do. 

I also already have AD2 and I upgraded to Melodyne Editor so I will take your advice and just install the Anderton FX Chains and Help Files. Oh, and REmatrix Solo which is also new.

Thanks again for the input. (and Karyn for the link)

maybe this will help http://forum.cakewalk.com/FindPost/3162662

Yes, it did.
Thanks Skook.

If someone already owns, for example, AD2, Lounge Lizard Full, Breverb Full, etc., are there any issues with things being replaced or just screwed up in general when doing an upgrade from X3PE to Platinum?

percepto
Still no clear answer and no bakers helping out?
Surely this is not a difficult question to answer.
I don't think it is too much to expect clear and precise instructions and I don't seem to be the only one with this issue.
I actually expected to be flamed for asking a question that has already been answered!
Help, please...

This is a valid request.  Clarity on install directions would be highly appreciated, even for something "straightforward" like the Braintree release.

You'll find there are indeed updates to Dim Pro and Rapture.

Well, I am  about 32 months late on this conversation.
I was searching to find out what the Classic FX were and how to use them, and it appears to me that the confusion is in how they are grouped/labeled
 
I found this
https://www.cakewalk.com/Documentation?product=SONAR&language=3&help=Plug-ins.2.html
to see the "Classic Plugins FX" - which makes it a bit of a hodgepodge as they are grouped but not easily identified.
That is an issue in how they are presented. There should be one name to represent them over time.
 
That may make the marketing team mad - but consistency in naming is key to having a solid product.
 
In my not so humble opinion.