No, it kicks in for as long as it wants not in *short bursts* Only time it will take it down a notch is if the TDP goes to high, which rarely happens. Heres how it works. At stock the i7 has a core clock of 2.66 and to achieve that it has a BLCK of 133mhz with a multiplier of 20x which = 2.66. Now when the cpu is active it activates turbo boost. If all 4 threads are in use than turbo boost comes in at 21x which = 2.8ghz, 2 threads its 22x and a core clock of 2.93 and with just 1 thread its a 24x it has a core clock of 3.2ghz.
Now of course not all apps are multi thread aware but turbo boost kicks in real hard for those, But most apps are multi-thread aware. Its more than a *slight* advantage. If there are coded the best way possible for multiple threads than its going to be twice as fast. Pretty much all apps are becoming multi thread aware.
The Speed is not decreased when all threads are maxed out. It never does that unless it starts throttling from heat problems. Sounds like your saying if all 4 are maxed than its going to be at 2ghz or something.... If they are all maxed than it will run at 2.8ghz. Other things wrong with this statement but i got the important ones.