Lucky, I love the mara. I have spent 6 months working in a camp there and have been back quite a few times. So have a little experience.
First off i assume you are flying down to the mara with safari link. There is a very strict 15kg on the flights, for all luggage including cameras. So weight is an issue. I travelled last in May with the owner of one of the camps and we were the only people on the plane and I still paid excess. If you are going at peak season i.e. Migration and the flights are full I have known it that people have had to leave clothes etc at Wilson. Which is far from ideal.
Which camp are you staying at? All Kicheche camps are in conservancies which means you are allowed off road driving which means you get much closer to the game than in the reserve itself. On my last visit I would say that 75% of my shots were taken with 5d III and 70-200 f2.8 IS II. I had 1d III and 100-400 as my other lens. I am planning on going back out there to work next year and when i do i will replace the 100-400 with the 300 f2.8 IS and that would be my gear when guiding.
As you say the majority of the photography is done early morning late night, but it gets light extremely quickly and then you are into golden hour when the light is incredible. In the morning it will be light enough for f4 very quickly. The only time from my experience you need the low light capacity is on your way back to camp at night if you see something. You have to be back in camp just after sunset so there is no night drives so your really low light situation is negated. Also the ISO performance of the 5d mark III would allow you to compensate and the fact you have the 300 f2.8.
My advise is stick with what you have got. the 300 2.8 is a perfect lens for the mara you have your low light capacity in this lens and with the extenders provides you with the length for when you are in the reserve and cant quite get as close to the game as you can in the conservancies.
However, there is no denying the image quality of the 2.8 II. I love it and in the mara it is my go to lens. if you are looking for a reason to get it and have the money it will be a benefit and you also have the flexibility of a zoom which matches the image quality of the 300. Do you need it and would you be £1700 better I would say no when you have the 300 2.8 already. But that is only a question you can answer. You have spent some serious money with a length of stay that long, you dont want to be sitting there with a leopard on a tree in the fading light and it be too close for the 300 wishing you had bought the lens. You can always sell it on after the Safari.
Have an amazing time! I am very jealous 12 days is an amazing length to get some incredible shots. If you are in the Mara North conservancy head to leopard gorge at sunrise as the first light hits it magical. Also try and get your guide to take you out while it is still dark and then watch the first glimmer of light cross the plains as the light first comes up. The mara is a truely special place. Enjoy
