You're probably correct that any turbo effect when multiple cores are in use is less than the quoted single core limit. In my limited experience, I found nearly enough turbo boost on a 2.2Ghz CPU to match a 2.7Ghz CPU when all cores are in use.
The "safety" aspect is handled by the combination of a hard limit which may depend on base clock speed and number of cores active, plus real-time thermal feedback. Due to the thermal limiting, there will be sample to sample variations and possible dependence on the cooling system. It's probably not surprising that you can't get quotations when some of the factors are external to the CPU.
