For everybody who has read Isabelle's article Supporting a large bust
The conclusion is that when making corset for a large bust (cup size D and more) busk can't sits fairly flat against the sternum.

I am making a corset cup F and I was planning to use Solid Wide Steel Busk to add additional support. But I am now wondering is that a good idea, or this busk will protrude even more?
Should I just use regular Flexible Steel Busk?

I think it depends how stiff the wide busks are. A wide busk would definitely separate the breasts even more, if you try to get it to lie against the sternum, which is probably not desirable with a large bust. You could pick up one of the wide busks you're considering and try it in a fitting mockup to see how you like it.

You are right. Those busks are 2,2cm wide. They are quite stiff. Now I am thinking, the surface they cover is 4,4cm wide in a stiff steel and they are not much flexible. Big bust might make it bend in the underbust area and this might be too uncomfortable for somebody who is not used on wearing corsets. Or they might push up the breast too much (and that is what I am trying to avoid). So I will stick to a standard busk and put the additional flat steel bones behind them for additional support. Sounds like a good compromise.

Thank you bookwyrm

You're welcome!