Stacking them has always been what I'd prefer to do, Roy - it means less outlay, and (slightly) less hassle swapping TCs in and out.
As it happens, today's trip didn't really work out - lots of SEOs about, but I couldn't get onto them, and the long-staying GG Shrike didn't show - but an obliging little Robin followed my around for a little while, usually too close to focus on.
So not much to show for the day, but the attached suggests that I'm gonna be very happy with the sharpness and the stabilisation: it's a stacked TC shot (for some reason the Kenko 1.4 isn't being seen by the camera, and having it in the loop causes some over exposure), but this is 588mm handheld (940mm counting the "crop factor"), 1600 ISO, 1/50, with a lot less sharpening than the 100-400mm used to get - I'll need to work on my sharpening regime, because everything I do as a rule seems to be too much now.
This is at f/6.3 (the camera put it there, not me), but things are as sharp at f/5.6.
I am extremely chuffed by this as a first "real world" example.
As an aside, this is converted/resized/denoised in Lr4 beta - I'm very impressed with that too. The only Photoshop contribution is my usual selective sharpening.
Oh - another thing: I was worried that the TCs might muck up the bokeh, but so far no evidence of that.
