Turbo Boost can certainly apply to all cores- the limits you hit are TDP and time based, there, not strictly thermal.

So, for example, my old laptop CPU would clock itself up to 2.7GHz on all cores... well, okay, it was a dual core, so that's not saying much, but still. But it'd only maintain that boost for a few seconds- under sustained load it dropped down to 2.5. This wasn't because of thermals, but rather because 2.7GHz was a Turbo Boost frequency, and once the PPL timer runs out...