It is Jonathan's Fault.... :)

[neuroanatomist]

The RF 100-300/2.8 does great with the 1.4x TC, and I find the resulting 140-420/4 very useful for field sports, etc. Performance with the 2x TC is not as good, IMO. Still quite good at 600/5.6, but my EF 600/4 II is better, and the RF 100-500 at 500mm is nearly as good as the 100-300 + 2x. I would not use the 100-300 + 2x for birding, for example.

On the wide side, unless you really need f/2.8 I'd look hard at the RF 10-20/4. If you're combining it with a 24-xx zoom anyway (and the 24-105/2.8 is excellent!), the 10-20 is a really fun lens...go ULTRAwide or go home. I do still use my RF 14-35 for wide shots around the house, but the 10-20 has replaced it as a travel UWA zoom.

[Busted Knuckles]

Very helpful.

the RF 100-500 at 500mm is nearly as good as the 100-300 + 2x.

I certainly use the 100-500 for birding with pretty good results and am very impressed w/ its sharpness. As JH pointed out the bokeh/subject separation is not the same a f4.....

I like the idea of the ultra wide - hmmm.... more fun to exercise my GAS self management skills.