If I understand it correctly, managing CPU resources is a job for OS, not application.
Application will request a launch (aka fork) of a new thread from OS, which then decides on which available cores to run them.
So I would say it is not possible to define how many cores a multithreaded app will use.
I'd say it will use all of them, provided OS permits.
Even iTunes spins off 30+ threads easily, so it can't be allocated a physical CPU core for each thread.
While you're technically correct, your answer doesn't help the OP much. Keep in mind that an application can determine the max amount of cores it uses (invariably) by simply not using separate threads or using less threads than there are cores. 1 thread = guaranteed 1 core (though not necessarily the same core all the time).
A hyper threaded app like FCP X will use all cores when necessary. When you're doing something not so heavy, it'll only use as much cores as is necessary. If you're exporting or rendering, then it'll use all 8 threads in a quad core i7.
Dual core i5s and dual core i7s, along with desktop quad core i5s only have 4 threads.
In Activity Monitor, every 100% of a process is one thread. If an app is using all 8 threads, it'll be using over 730% of CPU power.
This is a good measure to see if an app is using the full potential of your CPU. If you have 4 cores/threads and an app uses between 300 and 400 cpu % in Activity Monitor, it is using all your 4 cores. And so on for more cores. You can't predict, however, if an application will use MORE than those cores if you give it to them (but you can search online for that information, mostly).
So, does any modern application have the potential to use all the cores and threads?
Potential? Yes. Do they actually use it? Sometimes.
Take Word, for instance: it might use all the cores on your computer when you try to crop or adjust an image in its UI. But does it actually benefit from jumping from 4 to 8 cores? No way. It's an app where
the computer is waiting for you to type stuff into it.
iPhoto on the other hand? It is waiting for you while you browse, but when you actually try to edit a photo,
you will be waiting for it. These are the kind of apps which you want to look if they'd benefit from more cores. From your list: iPhoto, Aperture, and iMovie. They do benefit from more cores.
To your question: the 15" will be faster in iPhoto, Aperture and iMovie, but on the rest of it, you probably won't notice a difference. I have the 2011 13" (2 cores, 4 threads) and it does fine by today's standards.
