...
Can someone tell me if the following applications use multithreading or not? Is there any benefit to the i7 for simple tasks like web browsing and email if you don't use multithreaded programs?...
Those apps are all heavily multithreaded, however this by itself does not indicate whether i7 hyperthreading would have any substantial benefit.
You can see how many threads each app uses by looking at the "threads" column in Activity Monitor. I just checked FCPX on a simple project and it's use 63 threads, Premiere CC uses 43 and LightRoom 38.
A multi-core CPU is not limited to running the threads of a single app. It can run any mixture of threads from multiple apps. A typical Mac may have *thousands* of threads. Many of these are OS related which must run for your app to get work done.
However most of these threads are in a suspended or non-runnable state. They are typically waiting on user input, or a synchronization event, or I/O. Multiple cores only help when threads are in a runnable state. Even in a runnable state they may not have the characteristics which favor hyperthreading. The OS X thread dispatcher apparently tries to detect this and in some cases you'll only see every other virtual core scheduled, e.g, four cores in use on a hyperthreaded CPU with eight available virtual cores. This is because in some cases using all available virtual cores will cause "CPU cache thrashing" and actually degrade performance.
You can turn hyperthreading on and off with the free utility CPUSetter. You can Google for that but use at your own discretion. In my testing, hyperthreading improved FCP X export performance by about 30%. However it did nothing for Lightroom import/export.