Hi and welcome to POTN,TR4, Autocross 2009 Triumphest, San Luis Obispo, Calif.
If you are panning with the subject, try using IS Mode 2. This mode corrects movement in one direction only (the vertical axis, no matter what orientation the lens and camera are in). However, birds walking are not a fast moving subject, so you aren't likely to get background blur effects anyway, which is the primary purpose of IS Mode 2. It's most often used with rapidly moving subjects, for blur background effects such as this....
EF 70-200/2.8 IS lens at 200mm and f16. EOS 50D camera at ISO 100, 1/50 shutter speed. Handheld, avail. light (no flash).
Mode 3 is "instant IS", doesn't operate until the very moment of exposure. My lenses are older models that don't have this, so I cannot say anything about the nuances of it. But it shouldn't be needed in an situation such as this. AFAIK, it's basically the same as Mode 1, but only operates after you have pressed the shutter release. Mode 1 should work okay, though, if you are slowly following the subject as it walks.
You mention using a 2X teleconverters along with a fairly low ISO and a a slow shutter speed... how slow? Could the blur you are seeing be subject movement blurring? If faster moving parts of the bird, such as it's legs, are more blurred, then it's likely you are just using too low a shutter speed for the subject. IS can't help with this.
This is some strong subject motion blur, due to slow shutter speed...T. Wright & Herke B Hossmoor Dressage Show 2009
EF 300mm f2.8 IS lens at f5.6. EOS 50D camera at ISO 100, 1/25 shutter speed. Tripod with ballhead & gimbal mount. Ambient light, no flash.
Notice how the horse's hooves are more blurred above, they're moving faster than the rest of the subject.
The other possibility is that effective 600mm at f5.6 makes for pretty shallow depth of field, particularly if your subject is pretty close. Some focus error along with shallow depth of field might be another reason for image softness.
In the image below, the horse''s face and neck are within the plane of sharp focus.... just barely... while the rider's gloves and everything else goes soft. The closer the subject, the shallower the depth of field...Sadie... after the dance
EF 300mm f2.8 IS lens at f2.8. EOS 30D camera at ISO 640, 1/500 shutter speed. Tripod with ballhead & gimbal mount. Ambient light, no flash.
The only real reason to turn off IS when a lens such as yours is on a tripod is if you want to conserve battery power. IS uses a little power when it operates. Not a lot though. And, when the manuals (which aren't online yet for download, AFAIK... I looked at the US, Canadian and UK websites) mention using the lens on a tripod they are referring to using it locked down to prevent any and all movement... This is not
referring to using a gimbal mount for tilt control and loose panning axis to following moving subjects. I've been using IS lenses this way for ten+ years and I practically never turn off IS.
These lenses, the ones with the more advanced forms of IS, will turn it off themselves automatically, if it's not needed.
On very rare occasions over the years, I've seen IS not work properly... it appeared to add more blur, rather than reducing it. It might have been "user error", me shooting too fast. I try to track moving subjects, locking on AF and spinning up IS well in advance, than after a moment gently press the shutter release to take images. Doing this means anticipating and/or concentrating on keeping the AF point right on the critter.
Depending upon what camera you are using, your focus mode is another thing that can get you in trouble and cause soft shots. First, be sure you are in AI Servo. That's essential with moving subjects. Next, I use Single Point the majority of the time, along with Back Button Focusing
. This leaves me in full control of where the AF system focuses... so any mistakes are down to me.
It would be very helpful if we could see your images, with the EXIF intact or noted separately. If you can't post an image here yet because you just joined, perhaps you could give us a link where we could go to look at it.