It's not like turbo increases power usage too much since it doesn't increase voltage.
The multiplier is locked on the current Lynnfield CPUs so turbo mode can only work by increasing the cpu base frequency which you could call clock speed. You may not agree but by any definition that is overclocking. While voltage may remain constant there is a significant increase in the system power draw, which can only arise from an increase in amps.
So turbo works differently on Lynnfield than Bloomfield? AFAIK, on Bloomfield turbo increases multiplier, you get a one step (133Mhz) increase in frequency. I would suppose it works the same on Lynnfield, like in your following example of 666Mhz is 133Mhz x5.
For the 870 for example the increase from turbo mode is 666Mhz, i.e. it rises from 2.93Ghz to 3.59Ghz. You can also of course manually overclock. One reviewer, Guru3D, overclocked an 870 to 4.1Ghz. This increased power draw for their system at idle by 13w, at load it was 133w. So on that basis turbo mode could result in about an 80w increase in system power draw. All these numbers are AC at the wall. Even allowing for that, these are significant figures which cannot be simply ignored.
Obviously overclocking to 4.1Ghz does not happen with default voltage so the numbers here are invalid. Additionally, turbo doesn't increase frequency on the 870 to 4.1Ghz even if you're using only one core. AND, the consumption of one core is not the same as four cores running the same frequency.
As to a Lynnfield being a fast Dual Core and a slower Quad Core. Not really. What you really have (with the 8xx Lynnfields) is up to 4 actual cores and up to 4 virtual cores. So faced with a demanding single threaded app a Lynnfield will put 3 of its actual cores into effectively a sleep state, ignore the virtual cores and concentrate system power on just one core. This can include implementing turbo mode, yes on just one core. Depending on the app, it will then bring in further cores up to the actual maximum of 4 when it will start to deploy the virtual cores. If turbo mode is turned on it will implement turbo mode before bringing another core into use.
I don't understand, you just stated why I called it that. Should I have said fast single-core? Running one thread on 870 results in a 3.60Ghz on that single core, bring on another thread and we have a dual-core running at 3.46. 3/4 cores and we have 3.20Ghz. And all of this IF the cooling is sufficient.