BogoMIPS are calculated only once, during system startup. they are used for internal calibration only. they are not directly related to anything else. also, they are not recalculated when you 'overclock', but non overclocked values are shown.
you can see current ARM clock with one of following:
vcgencmd measure_clock arm
be prepared, ARM core freq is not exactly the same as x86 core clock. they mean relatively similar things, but real changes in system productivity are different..
speaking for BogoMIPS, i love this quote from one of their pioneers:
The reasons (there are two) it is printed during boot-up is that a) it is slightly useful for debugging and for checking that the computers caches and turbo button work, and b) Linus loves to chuckle when he sees confused people on the news.
( 1993, Lars Wirzenius )