So I’m just gonna write all this out because this is a big investment and this community seems to be pretty helpful! I’m strongly considering purchasing this TC if it exists along with the 200-500 f4 lens upon release. I am an ardent birder and photograph exclusively birds. I have graduated over the years to “better and better” equipment and currently have the R5 with 100-500. It’s a very good setup but it does have some limitations. As I get further and further into photography I seem to be becoming less of a birder to some degree and my outings seem to be more centered on photos as opposed to number of species seen. Not always but most the time. I am not wealthy and work as a social worker but would buy the lens and tc as it brings me great joy, helps me stay sober doing something positive, and helps balance out a stressful job and that is hard to put a dollar amount on. That being said if I spend 15-17k on a “hobby” I’m gonna have a few questions. With my current setup I am able to bird pretty easily with a cotton carrier for my camera and have never really felt fatigued by the weight of the camera. I realize with this new setup it may hinder my movements if I hand hold and i am not sure if a cotton camera will support the weight. That being said I think the reach I will gain not to mention the lower F stop will really lead to greater photos and more keepers. Currently I only average a few keeper photos each outing but with this new setup I think this will increase and if I am getting photos I like now with the 100-500 I imagine the f4 500 will look even better and low light situation will also improve. Also with the 1.4 tc engaged I will be at 5.6 at 700mm which will be great as well. And at 2x tc it will be f8 but 1000mm !! The f stop May increase but that doesn’t seem that bad for such an increase in range. For ID shots of distant birds or ducks, raptors, and shorebirds that type of range would be awesome ! I am looking forward to the fall and can’t wait to purchase these items. Although I’ll have to finance a portion so the monthly payments will suck !!
As
@Hector1970 says, a future 200-500/4 will likely be a heavy lens and you should consider the impact that will have on your photography. Like him, I also have a 600/4 II but in my case I frequently use it handheld. For winter raptors where I'm in a fixed position for a long period of time, I use a tripod and gimbal. But I also walk/hike with the lens on a shoulder strap (BlackRapid Sport-L).
Another consideration is how often you zoom out while using your 100-500. For me, I use my 600/4 with the 1.4x over 90% of the time...my personal bird photography would not benefit significantly by being able to zoom out. If you typically shoot at the long end of the 100-500 and you then crop in further, you might consider whether the existing RF 600/4 would be a better choice for you than a 200-500/4 (the former may be both lighter and cheaper).
Cost is another issue. Not my place to tell you how to spend your money, personally I would not finance a lens for my hobby. But if you're going to spend the money and if you have the time, you might consider which would be better...a new, heavy lens to replace your current already very good setup, or a few trips to excellent birding locations. Bosque del Apache, Alaska, Costa Rica, if you're in the US you could likely spend a few days at all three for less than the cost of the 200-500/4.