That's not quite correct, Slobodan. It allows you to use either a twice fast shutter speed or a twice lower ISO. Using both together would result in a serious underexposure. 
However, if one is comparing an MFT Zuiko 40-150/F2.8 lens with a FF 70-200/F2.8, there's no doubt that the 70-200/F2.8 is more useful in some respects, and the MFT lens more useful in other respects, particularly in its characteristic of providing more DoF at a given F/stop. One source of confusion in such comparisons results from the tradition of always using the full-frame standard to describe lens focal length. The smaller format always has the DoF advantage, and the larger format always the 'shallowness of DoF' advantage at a given f/stop and equivalent focal length. Whether one or the other is considered an advantage or a disadvantage depends on one's style of shooting, and/or the requirements for a particular shot.
In full-frame terms the Zuiko 40-150 becomes effectively an 80-300/f2.8 (excluding DoF consideration) so one might think the Zuiko lens has the advantage of more reach. But that's illusory.
One advantage of the larger, full-frame format is that one always has the option of cropping the image to a smaller format. One doesn't have the option of increasing the 4/3rds format to a larger size. If one takes these options into consideration, the FF 70-200/F2.8, in 4/3rds format terms plus full-frame terms, becomes a 70-400/F2.8. The difference between 70mm and 80mm is not that great, but the difference between 400mm and 300mm is quite substantial.
Also, if one wants to achieve the same shallowness of DoF with the full-frame lens, using the same aperture as used with the Zuiko lens, one can sometimes do so by choosing the appropriate focal length then cropping the full frame image to 4/3rds format, but this approach would not work at the wider focal lengths.
Sorry if such reasoning causes some people's heads to hurt. It's all clear in my mind.
My apologies Slobodan. When I first saw your post it was 'twice fast shutter speed and twice lower ISO'. When I later posted my reply, I failed to notice you had amended your post. I must be getting old. Wink
Ray when I read the comment in blue I had a good idea that it would come back and bite you in the a.....
It happens to us all.....except Slobodan?
