Hallo Roy
I hear what you say. I have already switched to IS mode 2 for BIF.
Firstly, I used this with the Mk3 1.4 tc attached. The results were OK. AF was pretty quick but I think that I have been spoilt by the sharpness with the naked 100-400 Mk2. Whilst the snaps were adequate, I think that I am probably better off cropping from the bare lens. Although I have a tripod and monopod, I am not really a hide bloke as I like to walk around and am not really happy with a long period in a hide and possibly surrounded by others. I do like people but I prefer my birding, not necessarily alone, but relatively quietly. I am not really a miserable old beggar, honest.
Then I again tried the tc on birds in a bush and was much more happy with results. I think the tc will be a handy tool to carry with me should the need arise rather than as a permanent fixture on the camera.
Yesterday, from the garden, I tried BIF with the IS switched off. I could not determine that the AF was any faster than with it switched on. Probably me but I thought that MY results were not so good as with the IS switched on. I note what you say about 'big super teles' but the slightly heavier Mk2 100-400 is as much as I want to tote for five miles and still use with a degree of comfort. I need all the help I can get! It's alright for you youngsters. To echo a point made in a previous post, to keep switching the IS on and off is OK if you remember which setting you currently have. To check the IS status takes time in itself and could easily result in a missed opportunity however the fast the AF is supposed to be - how many times have I rued using the wrong settings for a shot without adding to the variables and confusion.
I did have chance to test the MFD and was pleased to get sharp snaps of a bumblebee on the garage wall. My Mk1 combo was great for dragonflies and butterflies that were not within macro range - most of them, in my experience.
I will try and post a very few illustrative shots but I still have to change image sizes.
Colin