New findings: I don't know what it is yet but the song I upsampled that I was so happy with sound correct but one track, the pedal steel is way down in level. (I know, that's probably why it sounds better).
I checked the source multitrack and the steel is at the right level so something is going wrong here during upsample export . Maybe I just triggered and purposeful anti-steel condition in the up/down sampler code - I dunno.
 
I'll tell y'all what fixes it.
 

Well if I turn off upsampling on the EQ that is in the steel's fx bin the steel level returns to normal after export.
The steel track has other upsampled effects on it that don't seem to be adversely affected by upsampling.
 
Crazy thing is, That is my goto eq and is placed all though the file on many tracks that don't seems to be affected this way.

Add SI Strings to the list of plug-ins that benefit from oversampling. I rendered a string part on a song tonight, and I could definitely hear the difference in sections where there weren't lower frequencies to mask the distortion. The highs also seemed a bit less dull.
 
The way I tested this was to group mute buttons on the VI track and rendered track so they worked oppositely (i.e., if I clicked on a mute button to mute, the other unmuted. If I clicked again, the button unmuted and the other muted). After matching levels, I closed my eyes and clicked on one of the mute buttons really fast - so many times I lost track of which mute button was on. Then I continued to click, but slowly. When I could tell that one option definitely sounded better than the other, I opened my eyes to see which track had been muted.

Same with SI Piano from what I can tell - there is a difference in the high end. 
 

How do I upsample the ProChannel Modules...?
 

From the registry, look up the clsidPlug value in the VST Inventory for the plug-in.
Add a line at the bottom of Aud.ini in the PluginUpsampler section. The format of the entry is
{clsidPlugValue}.EnableFlags=1
to turn on Upsample on Render. To disable either remove the line or set the value to zero.

Thanks scook...

Please report back if you hear that upsamping makes a difference with particular plug-ins.

I recently tested up-sampling with Addictive Drums 2 and thought that there may have been a positive effect on the sound - mostly in the highs.
 
Can anyone confirm that AD2 benefits from up-sampling or was this a case of wishful thinking?

btsabq
Can anyone confirm that AD2 benefits from up-sampling or was this a case of wishful thinking?

I heard a difference as well. VERY subtle and only on some cymbals, but it was there.
 
FWIW I've been finishing up my next album (for those who don't know what an "album" is, in the distant past musicians would often put together a collection of songs intended to provide a cohesive listening experience, sort of like an "audio movie"), which means a lot of mixdowns. Upsampling definitely makes a cumulative difference if a project has several elements that benefit from upsampling. It's subtle, but every little bit of increase transparency helps...sort of like blowing the dust off a painting.