good laptop (very recent) with a high end ati mobility chip, you can get some good results.
I have an msi GX660r that has a 5870m chip, and it gets between 192 to 200 mhash a second! (after some tweaks) this is
a considerable amount so why not have it going when I'm not using it... like over night.. it ads quite an amount of shares and
is far more efficient that some of the major graphics cards in the 4800 series (better than my monster 4850 X2 which is the size of a boat and only get 150mhash but sucks power down like jughead does hamburgers and shakes).
That being said, the msi I have has very good cooling with multiple fans and even a fan turbo button. Many laptops have extreme issues with heat. I have left the laptop running stable now for a few days and it's considerably added to my shares
without using that much power as a desktop. So if you have a 58xxm or higher it's probably worth it... other than that... probably not.
How are you getting such a high hashrate? My 6750M only gives me 110 mhash after overclocking it.
The 6870s both the full card and the 6870m are much slower than the 5870s series.... just because a card has a higher number does not mean it is 'better'
So for that card you are probably getting the best you can on a laptop which is usually about half what you would get on a desktop...
you might be able to tweak by running AOCLBF 1.74 and running the phoenix phatk mod... but 6870s are just not as optimized for hashing as they are for games.
https://en.bitcoin.it/wiki/Mining_hardware_comparison