no two chips clock the same. i have a 4850 that clocks to 750 and is rock solid stable. it is on a generic pcb, and from what i gather most 4850's, even with voltage adjustment availible cannot reach such speed. in other cases they can reach this speed, even surpass it. if you get a poor clocker, chances are that you are throttling on the gpu or pwm's, or even dare i say, a badly put together vBios. but one of the biggest impacts on getting a bad clocking chip.... is that it is actually a bad clocking chip.
tl;dr all chips clock differently. ymmv.
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