....... but f/2.8 on their CX sensor is equivalent to f/7.6 on FF...not very impressive....
I never knew there was a correlation as direct as that. So if I have a 24-105 f4, is that like f6.4 on a APS-C sensor; or do we need to consider other factors too.
Just learning here... cause then my f4 lens on a 5D is not too bad compared to his 70-200 f2.8 II on his 7D (f4.48 equivalent) or should I consider other factors too.
Basically, yes. But to be clear, the crop factor applies to aperture in terms of depth of field for equivalent subject framing...so, f/2.8 on APS-C gives you the same depth of field that you'd get at f/4.5 on FF, for the same subject framing. The reason is distance - the crop factor means a narrower angle of view, so to match the framing you need to be further away with APS-C, and that extra distance means a deeper DoF.
Many people don't realize this. A great example is people who state they won't upgrade to FF until the 24-70mm has IS, because they want a FF equivalent of the very nice EF-S 17-55mm f/2.8 IS. What they don't realize is that such an equivalent exists...and is actually better spec'd - the actual FF equivalent of the 17-55mm is a 27-88mm f/4.5, so the 24-105mm is wider, longer, faster, and still has IS plus on the 5DII it will out resolve any APS-C sensor.
The effect of crop factor on aperture does not apply to exposure, though with the better ISO performance of a FF sensor, you can bump up the ISO to compensate for the loss of a 1.3-stops of shutter speed.