Took my newly acquired 100-500 for a bit of testing to a local site where some Snow Buntings have been over wintering. Carrying the combination is a wonderful lightweight relief from my 500mm f4Mk2 with a DSLR, especially my now departed 1DX2. I have attached a Wimberley QR plate to create a better hand grip/carry handle at twice the length of the lens foot ( 4 inches in total) as I don't us the carry straps that either camera or lens come with.
Unfortunately my ordered RF 1.4TC didn't arrive in time for the shoot but it has now so I'll test the combination later. No doubt you will by now realise that if you attach the TC to the lens you have to first extend the barrel to 300mm otherwise the TC won't fit. You now have an effective 420-700mm lens so as long as you are happy with that all is fine, however one thing I hadn't considered is if you want to leave the TC attached and stow it away in your bag, or even have it hanging from your carry strap, with the lens hood attached it is considerably longer than it was before..an extra four inches long in fact. You might like to factor that in to your decision about a camera bag amongst other things.
One of the known problems of zoom lenses with extending external barrels is their weakness to taking in dust. The L class lens does it's best to prevent this I'm sure but my 100-400 shows slight evidence it inside the lens and no doubt in due course so will the 100-500. It doesn't show in the shots you take but it is there. Having the barrel permanently extended when you have the 1.4TC attached will increase the likelyhood of it collecting dirt and worse still, salt in the wrong environment. So beware, maybe taking a cloth to wipe it down before taking it apart might be a good idea, especially in maritime locations.
Anyway, here's my Snow Bunting on a rather dull day. ISO 800, 1/800th f7.1 at 500mm. An approximate 70% crop.
I have run the final image through Topaz but at ISO 800 there is no discernible difference to the final image.