Why, do you think Sigma would release a bad lens? The only people worried about lens quality these days are maniacal pixel peepers. I need 20 on the short end, 28 is too narrow, and so is Tamron's new version with 25. These lenses will have different strengths and weaknesses. One will have better CA, another will have better corners. They're superzooms, there will be tradeoffs. But no lens company in their right mind would release a crap lens these days, too much competition from the cheap Chinese makers. I'm surprised one of them hasn't already climbed on board. 

Do you know anything at all about photography, cameras, etc., or was this a purchase because of the trip? If this is your first entry into photography, ignore everything anyone tells you about settings, it will just confuse you. Put the camera in Auto and shoot, you'll be much happier.  Conversely, if you know about such things as shutter speed, apertures, ISO, Depth of Field, etc. etc., then you should already know that there is no such thing as a 'best setup' for an outing that will vary a widely as a safari. You will encounter everything from action to static shots, landscape, light, dark, the entire gamut. If it we me, I'd shoot in manual and make adjustments for each setting.  My wife and daughter went on Safari a year and a half ago. My daughter took her Nikon. My wife is not a camera person so I sent her out with a Kodak Easy Share 16MP something or other set on auto and told her to just press the button. We have several of her shots framed in 8 x 10. 

Could be a good travel lens I guess. I'd wait until there are some reliable reviews.