It isn't a true macro no, and it is doubtful you'll get true (usable) f2.8 at 70mm. I would avoid it if I were you - the Canon version is worth the extra money. Read below...
http://www.the-digital-picture.com/r...ns-Review.aspx
If you're worried about low light, how about getting a cheapish prime lens in the middle (or upper middle) of that range. The nifty fifty would give you the low light ability, although you would miss the reach of the 24-105. If you have to, up the ISO and make sure you shoot RAW. Remember f2.8 is only ONE STOP better than f4, so that is the same as shooting at ISO800 instead of ISO 400.






