The EF70-200mm f/4L IS USM is an f4 but with IS would give you that same possibility of low light as an f 2.8. I know someone that is using it with great results. Also the EF 24-105mm f/4L IS USM is a great lens with the same features and professional results. What camera are you using? A full size sensor will give you true wide angle at 24mm but a 1.6 APS size would give you a 1.6 crop factor so you would get for 200mm about 320mm equivalent.
Best recomandation I have is get the lens you want like the (28-300), try and shot some pics and see if you like it, if not return it and get something else. Like I said before I am not a big fan od the extended range zooms. (I don't like noise and artifacts in my pictures, and I like to blow my pictures up quite a bit, therefore sharpness is a big issue with me)
You could do what some of the professionals do, get two bodies, and two lenses and do exactly what you want without any compromise.
