I have a 40D that is about 4 years old. We are going on a safri trip this summer. My current lenses are 24-105 4L and a 100-400 4.5-5.6L IS The f stops are not all that fantastic, even though they were not inexpensive. My question is should I be happy with my lenses as are and get a FF 5D(hopefuly new one is not THAT much more than the current) or look at getting a faster prime lens such as the Canon 24 f/1.4L.
My oher interest are primarily very still photos, landscape and people
I was in Tanzania back in 2010, and took a 40D too. I had a 10-22 and the older 80-200mm 2.8L with a 2x. Yes, you will need the the 400 range and more for birds and some other animals. In most cases, it will bright enough, meaning you will be able to use the lens wide open in the morning or at dusk and closing down and/or using low iso during the day. I do recommend an UWA lens - the 10-22 for the crop body does wonders, as the lens distortion is not the worst - not a bad as the 15mm F.E. on ff. You will need the wide angles for landscaping - yes, africa is big and with wide spaces, but even at 17mm not everything will "fit" in.
The 24-105(36-170 on 1.6x) will cover most of people and closer animals shots. The 100-400 will do a good job on most animals, except in heavy shade situations; so in my opnion, you need to cover below the 36mm range - hence the 10-22mm.
No question you will benefit with a ff body, and your kit still works very well with one, but in that case you would need the 17-40 or 16-35 lens.
IF you can, rent the 5dc or 5d2 with the 16-35 or 17-40 combo - too many chances to shoot, but probably not so many chances to go in safari every year
besides, you want to avoid lens changes while on the safari - bumps, shakes and plenty of dust all the time