I would keep the money for the moment. That or I would get the 300mm f/2.8L IS II. It is an excellent sports lens. Also, it is much cheaper and lighter that the 200-400 zoom.
Earlier in the year, I was on an Irish sea birds workshop. I was the only 400mm f2.8 L IS, mk I...and flippin heavy...but stunning pictures, even If I do say so myself
There was a 600 LIS II and a 500 LIS II, both very good.I think the 600 is a bit better than the mkI. The new 500 is increadibly light and easy to use. There were two 300 LIS II with TC's and again very light and easy to handle, sharp and beautiful optics. It lost a stop vs my 400 when focal matching but could move closer to the subject to get the same framing.
If I was going to do it again, it would be a hard descision between the 300II or 500II. My 400I was mroe flexible with TC's but heavier and a lot harder to handle when tracking large birds...even lugging it abot over the island was seriously hard work! I personally found the 600IIL a bit too long for my likes. I suppose I could chop my 400L in for a 400IIL