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I'm not sure exactly what people expect of a 64-Bit native app, and question whether those expectations are realistic.

Most 64-Bit benefits will come from RAM and CPU intensive tasks such as rendering. If you think that it's going let you smoothly handle scenes with several dozen Victoria 4s (all fully clothed, of course), you're going to be disappointed.
For example, it is not difficult for me to push my full 64-Bit native version of Vue 7 to the point where everything slows right down or crashes.

If you say so. I have Lightwave scenes using up to 7+ GB of RAM at render time - with 64 bit Lightwave, of course and nothing slows down. That would be impossible in a 32 bit app.
What are your system specs? RAM, RAM speed, motherboard model, CPU?

I also have one Poser scene that uses 5 GB of RAM, using Poser Pro and rendering with 64 bit Firefly. IMO, they did a pretty good job with it.
And I routinely ran my RAM up into the 6 GB range when using the Mudbox 64 bit demo.

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It would great if I could specify how many cores get used in each of these modes.

You can, in Windows at least. Task Manager>Processes - right click on process and choose "Set Affinity". A dialog which lists all your cores pops up and you can select which core does what. The settings don't stick after you close the program though.