The processor used in the early 2011 15" and 17" i7 models is the 2.2 GHz quad-core (2720QM) Intel Core i7 Sandy Bridge with 6 MB on-chip L3 cache, so they have 4 cores and 8 threads.
Apple's MacBook Pro performance page makes it clear that Hyper-Threading is now standard on all MacBook Pro laptops.
Turbo Boost is a feature that, when fewer than the total number of cores are being used, the processor can turn off the unused cores and increase the clock speed on the rest of the cores. This increases performance (the cores that are being used get faster) and can reduce power usage.
For instance, if you have the 2011 MacBook Air with the 1.6 GHz Core i5 (as detailed in this AnandTech article), the "Turbo Ratio" is set to 0047. The number sets the amount of boost by digit (4 cores = 0, 3 cores = 0, 2 cores = 4, 1 core = 7; this is a dual-core CPU, so 3- and 4-core mode is irrelevant). The number specifies the boost in 100 MHz increments above the listed clock speed, so if two cores are running, they can be boosted to 1.6 GHz + 400 MHz = 2.0 GHz and if one core is running, it can be boosted to 1.6 GHz + 700 MHz = 2.3 GHz.
To see what your system is set to, go to Console and open the kernel.log (or system.log in Mountain Lion*), then search for AppleIntelCPUPowerManagement
and look for "Turbo Ratios". The Mac I'm on now (a 2.0 GHz Core i7 MacBook Pro), for example, has Turbo Ratios set to 6689 for a maximum clock of 2.9 GHz when using only one core.
* thanks to @gentmatt in the comments below
Best Answer
performance per watt
You want to know the 'performance per watt'. In computing, performance per watt is a measure of the energy efficiency of a particular computer architecture or computer hardware. Literally, it measures the rate of computation that can be delivered by a computer for every watt of power consumed.
2.5GHz Quad-core Intel Core i7 = 21.05 pt/W
2.8GHz Quad-core Intel Core i7 = 33.4 pt/W
So, when only looking at the 'performance per watt' you would be better of with the 2.8GHz
Sources used: Wikipedia, cpuboss.com - 2.5GHz, cpuboss.com - 2.8GHz
note: not 100% sure if I picked the correct processor types.